If either Janet or David comes/come...

May I ask a question about the usage of "either...or..."? Please look at the following sentences:

If either Janet or David come, they will want a drink.

If either Janet or David comes, they will want a drink.

If either Janet or David come, he or she will want a drink.

Could you please tell me which of those sentences are corect? And which of them is the most commonly used? Thank you very much!