Surfboard theory says, center fin forward side fins back, but that's with a reasonable amount of toe in. The euphorias I've seen don't have much toe in, not sure what effect that has, but I'd try center fin fully forward and side fins as close to inline with it as you can get. Should make it looser but may have other effects you don't like.

I've sailed the Euphoria for 3 seasons now and have mucked around a lot with the fin positioning and fin sizes. I tend to ride with the full thruster setup with the 14 cm fins Marty supplies. I recon that the best set up is side fins all the way back and middle fin 2/3 to 3/4 of the way back in the box. I find with this set up you can slide the tail just a bit off the top which makes it feel a little looser. This flys in the face of single fin setups where having the fin forward does make the board looser. With 3 fins you will always have heaps of drive of the bottom but finding that little bit of slide of the top is good. I have this setup in both my 77L and 85L boards. If you go to the 2 and 1 setup then move the bigger centre fin forward to gain looseness with the 2 little side biters all the way back in their boxes.