Thanks guys for your answers!
Still, I would like to know how the system would behave if I connected all 3 cards.
I guess I would connect them on PCIE1 - GTX 670, PCIE3 - GTX 460 and PCIE5 - GTX 670, and connect only the 2 GTX 670 with SLI ribbon.
In this scenario, will the GTX 460 occupy all x8 and thus PCIE1 will use only x8 ?
when using a card dedicated to physx, it occupies all the x8 length?
Is there a way to configure the system such that both GTX 670 will run x16 while using all 3 cards together?