It depends how you are riding it and what your skill level is. 3 fins is going to be more stable on the board with the fins in the rear position. As you advance you might want to pull the rear fin to loosen it up. You can also move the fins up to a more centered position. This will make spins easier at the cost of a little stability.
The Illuminati is a really stable and fun board in the 3 fin configuration. I have only ridden it in a 3 fin setup, but have ridden a similar board, the Carbon Parks thruster in a 2 fin configuration with the fins moved up and that felt really loose, I imagine your Koal would feel similar. The is REALLY fragile. Be careful with it as it chips and crushes easy and wears easy in board racks. Take extra precautions to protect it and/or be prepared to make some fiberglass repairs along the line.