Nvidia and ATI design graphics cards, where as Palit, MSI and other companies manufacture them ( different manufacturers add different toppings too, like adding ketchup or hot sauce :P )
So if u want to use 3 monitors at the same time, 5xxx series designed by ATI has the capability to use 3 monitors at the same time. The GTX 460 which is designed by nvidia, doesnt have the capability to use 3 monitors at the same time.
The 5830 and 5770 by ATI has the capability to use 3 monitors ( the feature is called "Eyefinity" ). The 5830 is the closest competitor to the GTX 460.
But if u want the gtx 460 really bad and u also want to use 3 monitors, u need two gtx 460 graphics cards, running together ( Which is called SLI, coupling same graphics cards together so they offer better performance
) Careful though, coz SLI'ing two graphics card could be very alien to you.
So you are better off buying a 5830. Which manufacturer? i dont know!