Fins are a mystery. I've got a quiver of the things, and love playing around with them, and they can make so much difference to the way a board goes and then that all depends on the waves, board and how you are surfing, there's just so many variables, size, number, shape, flex, position, it gets confusing.
But generally, moving the centre fin back will make the board more drivey through turns, but harder to turn, further forward gives you a bit of sideways movement in the tail without needing to stick it on a rail so much.
You kind of need to keep the same fin area between a single or 2 + 1 setup, so a bigger single fin would be worth trying if you take the side bites out, but then a small single fin will slow it down in trim, but loosen it up.
The size of the fin area also depends on the width of the tail - more width = more fin area
But to be honest, it's best just to try loads of different fin setups, sizes, shapes, positioning, and see what you like.
Also, think about the flex of the fin, especially the centre - it can make loads of difference to the way a board goes - I love using a thin flexible in the tip, swept back fin on a longboard, makes it really snappy coming out of turns as it releases. But then big barn door fins are fun in small waves, because you can really push off them in a turn.