It depends on the size of your rings more than anything- each RD will have a rating for how many teeth it can handle (front+back). Check to make sure you're within the specs and you're good to go.

Sometimes you can get away with a smaller than rated cage if you are super careful not to cross-chain- IE, as long as you never use your big-big, you can use a medium cage on a triple. Not recommended though, because one brain fart shift of the front is enough to trash your RD.

PS: This is why I ride a single speed. Less **** to break and less decisions to make