A fence is a custom of a person who has made a vow.

Hello Rabbi,
I wanted to ask the Rabbi's opinion on an issue that bothers me, and I haven't found any organized answers to it in the relevant sources. Regarding the halacha that a custom practiced 3 times is considered a vow – a. What exactly is considered a custom for the purposes of this halacha: Does it depend on a specific intention, and does it also have to be a physical act or does a specific speech/intention in prayer also count as a custom and require permission? b. In some sources, the binding custom is defined as a "good custom" – what, in the Rabbi's opinion, could be a good enough definition for this boundary? c. Assuming that the answer to question b can be well defined, if a person practiced a positive custom and this became an undesirable burden on him later, would it be correct to say that this is not a binding custom because it makes it difficult, burdensome, and bothers him (and perhaps as a kind of regular vow that sometimes expires in principle in similar cases, of "I do not know this" and so on), or is such a custom binding because it is practiced in principle as positive?
Thank you very much in advance, Shabbat Shalom and Happy Holidays!