It depends on the person, but usually a good trend to follow if you’re unsure is to main the pronoun that comes first. Nowadays when people put several pronouns they tend to also prefer if you switched it up with one of the other pronouns once in a while. Doesn’t have to be all pronouns in one sentence but just switching up which one you’re maining right now.

For example, I use they/it/any. You can’t go wrong with they, but I would like if people used it and any other pronouns too. My friend uses they/she but prefers they so I call them they but pop in an occasional she.

And I understand the defaulting to they/them, but be aware that if it’s a trans person that has struggled to get their identity respected, not using the he or she that they prefer will make them uncomfortable and come across as misgendering. “Allies” that don’t respect your identity tend to use the they/them trick to avoid actively misgendering, but not actually call them by their real pronouns either.

Edit since I saw your other reply, if people say they don’t care they usually really don’t, or if they do care they’re being unfair. You could ask if they really don’t have any preference at all, but otherwise they/them tends to be a safe bet, maybe some switching it up.