On some of the Monkees episodes why was there sometimes only just three of them instead of all four?

Well, mike was the one who was usually missing for multiple reasons. Like in 'Hitting The High Seas, he actually did get sea sick, and I also know that he missed an episode because his tonsils were taken out. And Davy missed one because he was visiting his family in England. and... Thats all I know. :)

yes. In "Alias, Micky Dolenz", only Micky, Peter, and Mike were in it(aside from pre-filmed música videos), I think because Davys father was sick, so he took off time to visit him. Also In "Monkees watch their feet" Mike was not there, though I don't know exactly why.

I don't know why Mike went missing so much. I have heard that he wanted to take time off to do his own thing for a while. I don't know if that had anything to do with it o not. (I do like that, in one of the episodes where he was missing, they made it feel like he was still there por using only his songs and using old clips with him in the "Sunny Girlfriend" romp.)

As for Davy in "Alias Micky Dolenz," he dicho in the interview at the end that he went to England for his sister's wedding. "And I missed it. I was two days too soon."