2 +1 Board has three longlboard fin boxes...strange?

[AtlSurfGirl]

My 9' Pearson Arrow has the standard 10.5" longboard fin box in the center and a 7.5" longboard fin box for each of the side fins (all made by Fins Unlimited). This seems unusual to me (and my local surf shop). I want to keep a 2+1 setup, but can not find compatible sidebites. As you can see by the photo, the side fins are trash. Anyone know of a source or solution? Also, what about the FCS longboard box adaper? Anyone tried it out or know if it can be used to hold sidebites vs a center fin?

[IB_Surfer]

old school is more like it...