For a dedicated PhysX card, you should go cheap. Anything from a 9800GT to a gtx 460 is more than enough.
I have to say, though, that your setup seems overkill to a point of being couterproductive. Under 1080p, very few scenarios will require more than what one tahiti GPU can offer, and the rest of the time the other two present added complication and potential issues. I'm assuming you are going for eyefinity, otherwise that doesn't quite make sense.
A dedicated PhysX card is also of very little benefit, since this technology is almost never used, but since it can be had for little money, you might as well go for it. Bear in mind that crossfire+physx is an almost certain recipe for headaches, as seen here:
My two cents: PhysX is not worth it. Also, consider if you really need all that horsepower, because it comes with complications, and consider that in a year you will have lost half the money spent on those VGAs. I myself like to have a PC that can run anything with decent quality, but most of the time end up playing DotA, Baldur's Gate and even DOS games on a 500-dollar GPU.
PhysX is being used more and more, and with the latest APEX PhysX titles the effects are only getting more detailed and complex. If you want to play any of the highly anticipated upcoming PhysX games, that include AAA titles Call of Duty: Ghosts, Batman: Arkham Origins, and the Witcher 3, you're going to need a dedicated Nvidia PhysX card. There are, however, several items to consider:
1) You really need to think through your plan to go with Tri-Crossfire. Tom's Hardware did a review of two 7990's in Crossfire and found that, due to a heat vortex created between the two cards, it basically overheats and doesn't work. And then, of course, Crossfire is still plagued by the frame pacing problem in multi-monitors, DirectX 9 games, and OpenGL games. Playing Skyrim, for example, on Crossfire is no better than using a single GPU. http://www.tomshardware.com/reviews/radeon-hd-7990-cros...
3) With a Tri-Crossfire setup, you're going to need a decent Nvidia card to keep pace with your primary cards, and of course a PSU capable of running 4 cards. A card that is not powerful enough will definitely hurt your performance. The key for PhysX is a high CUDA core count and fast clock speeds. The latest GT 600 series is more power efficient than older models and packed with more cores at the lower end. A GT 640 or GTX 650 should be your minimum target (384 cores, same as a GTX 560 Ti). Here's one of the few reviews suggesting a GTX 650 Ti is ideal: http://1pcent.com/?p=169