Hi eRacer!
When it comes to triple-wide gaming with NVidia or AMD, all three monitors will have to be connected through one graphics card. So, you can't really have one card power one monitor and another card power the remaining monitors.
General consensus on the forums seem to be that, whenever possible, you should go with one more powerful card over multiple less powerful cards. I've personally run 2x Radeon HD 4870s, HD 5870s, and now HD 7950s and I really enjoy the extra framerates and graphic settings I can crank up with two cards. But, running two or more cards in Crossfire or SLI tends to cause issues. With AMD cards in Crossfire, you're more likely to encounter
microstutter. Not all games can benefit from multiple cards - AMD and Nvidia usually have to program special profiles for each new game to take advantage of multiple cards and less obscure titles may never receive support. You'll also be dealing with a lot more power being required and have the cards generate a lot more heat if you've got more than one.
Depending on what you want to spend, I'd recommend looking at the Radeon 7870, 7950, and 7970 or the NVidia Geforce GTX 660ti, 670, or 680. You could also save some cash by looking at the Radeon 6xxx series, but you should probably avoid going back to the NVidia Geforce GTX 5xx series because they require two or more cards in SLI to support triple-wide. Even though I've been in the AMD camp for years, I'd probably recommend that you stick with NVidia cards given that the Asus monitors you've got don't have the displayport connections necessary for the best results out of AMD Eyefinity.
I'll benchmark iRacing on 1x 7950 and on 2x 7950s and let you know what kind of performance you could expect from them. It should be fairly similar to what you'd see from the Geforce GTX 670 series. What games are you looking to run besides iRacing? Do you care about antialiasing?
Edit: Ha, I rescind my welcome. I just realized you've been registered on the forums longer than I have! It's been a long while coming, it looks like, but you'll absolutely love racing in triple-wide once you get up and running.