I want 3 chickens and i have a few questions???

3 chickens is not much....they dont lay every single day. Of course you can mix breeds. My parents had like 80 chickens and they were all different kinds...its only the males that like to fight which causes problems if you have a whole bunch (I would say 1 male to every 20 hens) but I doubt your getting males since you want eggs. That space is fine...as long as they can all move around without bumping each other. They do like to take dirt baths though so make sure you have a little area of dirt they can lay in.