Just the Deadpool movies, to get the majority, probably the Marvel Cinematic stuff to get some other inside jokes, but that’s a waste of time and not worth the effort, and if you want deep cuts, read the Deadpool comics. That’s how they make these movies; there’s stuff for the people that only watch the directly involved movies of the precise franchise, plus stuff for the total nerds. That way almost anyone can get something out of it. I like Deadpool, so I say watch the other two and then see this one. If after the first one you decide you don’t like it, don’t bother with the rest. It’s really that easy.