Let me get this right.
You want to use the BVision as gfx output and use the voodoo3 card in the mediator just as a DMA memory card ?
Why ?? the voodoo is quicker and has better 3D support.
Use the BVision to run MorphOS and use the Voodoo when you are in OS3.x.
You will never get the BVision running on CGX and Voodoo card running on P96 in OS3.x at the same time.
And i think the mediator uses the P96 voodoo driver to get DMA going. So in other words you have to have P96 running to be able to use the mediator with a Spider USB/100mbit NIC and so on.