There are many more things about which it says that someone who violates them, or someone who fails to do them, has no share in the World to Come. Both in that same source in Sanhedrin and elsewhere.
It’s really not only those three.
It’s also not clear whether this is a factual statement or an educational message to the people. There are transgressions that are clearly rabbinic, and yet very harsh things were written about them.
Until Maimonides, the attitude toward such statements was like aggadic literature, and even after him there were those who saw them that way. And even according to his view, to believe such things one has to adopt an Aristotelian worldview, which obviously almost no commandment-observant person today accepts. According to his approach, even existence after death (if it can even be called existence) is not automatic and depends on attaining wisdom, and apparently is not individual; and the commandments are only a means to enable exceptional individuals to attain the intelligibles, not an end in themselves.

I didn’t understand—according to your view, even apart from my question, God is not all-knowing?
If so, what about Psalms 139:
2 – You know when I sit down and when I rise; You discern my thoughts from afar.
3 You scrutinize my path and my lying down, and are familiar with all my ways.
4 For there is not a word on my tongue—behold, O Lord, You know it altogether.
5 You have hemmed me in behind and before, and laid Your hand upon me.
6 Such knowledge is too wondrous for me; it is lofty, I cannot attain it.
7 Where can I go from Your spirit? And where can I flee from Your presence?
8 If I ascend to heaven, You are there; if I make my bed in Sheol, behold, You are there.
I’d be glad if, in addition to the answer, you could point me to an article that discussed the subject (even just keywords would be enough), thanks.

Let’s say He does know everything—what exactly is the problem with that? He doesn’t owe me, you, or anyone else anything. The Calvinists believe He chose in advance who goes to heaven and who goes to hell, and no one has any way to change His decision. People are not equal. You could also ask why He made one person smarter than another. Why did He make another person be born rich or with exceptional ability in some field, another more beautiful, healthier, etc.?
Especially since wisdom, according to some medieval authorities, is the thing through which one acquires the World to Come. A person born with a low IQ, according to Maimonides for example, has less chance of attaining the World to Come. Even according to Kabbalah there are souls with a greater capacity to attain God than others. So where exactly does the morality you’re talking about enter here?
If here people suffer because of things outside their control, why is it impossible that they would suffer eternally and that too would be outside their control?

Maimonides even said: do not think that the whole world was created for the sake of man. Man is just another creature like the inanimate and the plant world, except that he has the capacity to grasp forms.

And as people already wrote before me, who says the Talmud here is describing factual reality? Maybe it is just a message to the masses so they won’t belittle the commandments, so they will believe, etc.?
No one can really know what there is in the World to Come and who merits it.
There isn’t even an orderly tradition—if there were, there wouldn’t be deep disputes among medieval and later authorities. And these are not point-by-point disputes, but about the system as a whole. And that too is derived from the knowledge and philosophical approach of the medieval and later authorities.
At most there is a tradition that there is something. What that something is—that is not known.

I asked Michael, but it’s always nice to talk with other people too.
A few points:
1. “He doesn’t owe me, you, or anyone else anything” — I pointed to a contradiction in the “goodness” of the biblical-Jewish God: an immoral act. It’s not that He owes me, you, or any other person anything—but rather that if what I wrote is correct, then God is not really good, or He does not really exist (and of course one can philosophize and empty the concept of good of all content by saying “God’s ways are hidden,” or appeal to mystery).
2. What you wrote about wisdom and higher chances of the World to Come doesn’t matter to me, because as stated, all Israel have a share in the World to Come (except for the three mentioned), so in any case even those with the “lowest” chances will get in, probably at a later stage (if one even accepts the assumption that wisdom is connected to the World to Come—I don’t).
3. I don’t understand what Maimonides has to do with this… but fine.
4. I don’t know, maybe it really is a message to the masses or something similar, but things are supposed to be taken literally as long as there is nothing indicating metaphor, allegory, esoteric meaning, or anything of the sort.
In addition, my whole question was under certain assumptions; obviously if you remove those assumptions, we can open a new discussion in no time.

He created the world for some reason. Not necessarily so that things would be good for people. Already here you can see that the world is not fair and not easy, and life is dangerous.
That’s your assumption. First of all, in that same source in Sanhedrin there are many more who have no share in the World to Come. There are also dozens of other sources listing additional people who have no share in the World to Come. Some over really minor things. Even rabbinic prohibitions, and even things that have no formal halakhic prohibition.
According to the medieval authorities, there are others too who have no share in the World to Come.
I brought Maimonides as an example. According to Maimonides, only through attaining wisdom can one attain the World to Come. A person can keep all the commandments and believe with the faith of the masses and still not attain the World to Come; on the other hand, according to the Guide for the Perplexed, one can attain the World to Come even without the commandments, and there is no difference between a Jew and a non-Jew.
According to Maimonides, all those beliefs are completely Aristotelian, and the belief required in them is not simple faith as people think, but intelligibles that a person must attain so that his intellect may unite with the Active Intellect. More than that: according to Maimonides, most religious Jews are heretics in this regard.
Likewise, what matters is not what a person says but how he pictures these things in his mind.
A smart person with a philosophical inclination will attain the World to Come, while his less intelligent friend will not.
Maimonides argues that the world was not created especially for man. People’s tendency—and apparently yours—is to think that we are at the center. But we are only part of a larger reality. Interesting that you didn’t ask about animals. For example, about the mouse that gets eaten, or the polar bear dying of hunger.
Good and evil here refer more to our relation to reality and the general approach. It’s like a certain ordinance or law that is said to benefit the public—not every detail is necessarily helped by it.
The Talmud is full of messages to the masses where you can see that the statements are not meant literally. They already gave the example of rabbinic prohibitions for which one who violates them has no World to Come, and medieval and later authorities already noted that this is not so. Or all the “whoever does A is as if he did B” statements, where it is obvious these things are not literal.
Likewise the descriptions of the World to Come, which most medieval authorities did not think describe factual reality.
It may be that they describe factual reality and it may be that they don’t. Even from the wording there it is hard to understand them literally. For example, they keep adding more and more things, or these are transgressions that affect the public, etc.

In any case, the claim itself might be true and it might not. If the Talmud said something and it turns out not to be correct, there would be no problem here. At most it was mistaken, or it was not referring to factual reality. If it is right, then that is the situation.
Like any factual question, it doesn’t matter who said it and why.

Look for the series of columns on foreknowledge and free choice.

“Not necessarily so that things would be good” — notice that I never claimed that in my life; my claim is that it is immoral for God to create a person knowing that his fate is hell. Try to focus on what I’m saying and not jump to conclusions.
Then you should have been clearer and said that wisdom can really determine it, and not just lower or raise the percentage a bit. But if that is the case, that’s acceptable to me, and I claim that that too is not moral, since God created the “stupid” people knowing that because of their stupidity they would not merit entry into the World to Come…
That’s not my tendency; I’m not even a believer. I see the world and the universe as slaves to indifferent laws of nature, but feel free to draw whatever conclusions you want.
You expanded into areas where I don’t have the faintest idea what I’m talking about, with medieval authorities, plain meaning, and the Talmud—I’m completely ignorant, so I’ll keep quiet there.
Usually it indeed doesn’t matter who claimed what and why—who disagreed with you? The difference is that the Mishnah and the Talmud have formal authority, so their words have significance whether they are right or not. Even as a believer I personally would probably reject those things, but that’s just a guess.

I’ll look. But regarding my question: assuming everything written is true, do you agree that this is an immoral act?

Obviously. To create a person just so he will suffer is an evil act. Why is this interesting? Hypothetical questions wondering whether someone who behaves badly is behaving badly. Strange and unnecessary.

This isn’t just a hypothetical question. It’s a common view in Christianity and Islam; the view also exists in Judaism, though to a lesser extent and with many internal contradictions, in my opinion.
And it’s not so simple—there are people who will try to justify it with free choice or nonsense like that… In any case, thanks.