Two issues...
...one, if it fits, it's going to be a REALLY tight fit. If you really want to try this, my suggestion would be to buy two GFX cards first and then check the fit on different slots between them. Then if it looks like you have the space, you can always get the sound card, check fit again, and go for the third GFX card.
...two, if it fits, good luck keeping those babies cool. My 8800GTS 512 ran just fine at stock fan speeds when there was just one of them in there. When I went SLI, I had to jack the fan speed up to 70-75% to keep them from crashing out because one card was basically sucking hot air off the @ss of the other. Adding a fan to move air around in the space between them didn't work. Although those are double-slot cards, single-slot cards will, in all likelihood, face the same problem.
If it were me, I would go 2-way SLI with a sound card. Onboard sound, although much better than it ever has been before, is still inferior to a good quality sound card. But SRCobra is right, this is only true if you have at least semi-decent speakers or headphones. Give the onboard a shot. I personally hate the RealTek audio most mobos use (including EVGA boards), but you might not.
In the end, I gave up trying to air cool my babies, and switched over to water.