Fin configuration

So our hyperlite broadcast 5'6" has three fins. When I got the board, I put on all three, but a couple of weeks ago I surfed my Dad's old Hyperlite Landlock (pretty much the same as my board, I've heard) and I seemed to be able to carve a lot easier on that board. The major difference is that board has two fins.

Are there any rules of thumb regarding fin placement? ie, what are the differences between having all three, just the two outside, just the one in the middle, or even no fins?