Firstly, cards which require three oceans, is this three oceans or more, or exactly three oceans? It's just that the other cards are very specific (say saying temperature 8 degrees or more) do you need exactly three oceans tiles to be on the board?
Therere no cards with exact requirements. If it is not a maximumrequrement it is a minimumrequirement.
Yes! As many as you want in player order.
Also, at the end if the game, you get one more opportunity to build greenery tiles, is this as many as you want in player order? Say if player A goes first, can he build three greenery tiles all in one go, before player B and C get the opportunity (this is important as you also get any or all placement bonuses)