Grohflex rough-in box but want to use 3 port diverter

Our plumber installed the Grohflex rough-in box and the marble has already been installed over the shower walls. We are now looking to purchase the shower fixtures and just found out the the grohflex trim that goes with the box only has 2-port diverters. We really want a 3-port diverter so that we can run the 2 separate shower heads at the same time. Is there anyway to do this? Can we use another manufacturer's valve and trim with this box? Any suggestions are greatly appreciated!

Thanks.

Beryl -