Some things to remember are the kind of center box fin you have.
If you're going single fin, you don't want to use the 7" fin that typically comes with the 2 + 1 setup (where the 2 are 2-3" side skegs).
If you try to put a 'standard' shortboard 3-fin set in a longboard, I doubt you'll get any (side) movement. They're meant for performance boards.
That being said, for single and 2 + 1 setups, there are a whole host of center fins to work with. I changed up to a wingnut cut-out. I've got a lot more side-to side movement, and the fat tail of the skeg sticks in the water and pivots my turns.
Losing the two side fins just makes the board looser (and therefore you'll probably need a larger center box fin). Also - it demonstrates why going with something bigger than 2"-3" for those side fins would kill your manueverability.