Depends on your system... you could list the specs and get a proper answer. http://www.tomshardware.com/reviews/p55-pci-express-scaling,2517-10.html
"Looking only at averages, the chart above makes it appear that adding a third card boosts P55 performance by nearly 8%. Yet the fact that it hurts performance in some games and offers no benefit in a few others means that we’d never suggest using the P55 PCH to host a third graphics card in CrossFire or SLI. Moreover, the X58 gains 25% by adding a third card and has no performance issues..."