I think it is a worthy consideraton for GTX470 3-way SLI, primarily you are running 3 monitors at 3x1920x1200.
Read this 3-way SLI GTX470 review. The limitation mentioned of the CPU would not apply to your situation as you are running a much larger resolution than the tested 1920x1200/2560x1600. I would expect a noticeable gain in framerate when going from 2-way to 3-way.
However, 2 areas need your attention:
You will need a more powerful PSU than your current 850W. I would go with either the Corsair AX-1200 or Antec's HCP 1200. At least, 1000W should be used.
This will probably be easily solved if you put the 3rd GTX470 into the loop. Even if not watercooled, putting the 3rd on the last slot of your UD-3R mobo should also work out.
I was looking for some benchmark test like this