Yes, look-up DP 1.2 MST
Just a single HD6990 (or 68xx) would support that, but you need a hub. However, that's why I've been waiting for DP1.2 on laptops.
If you want separate support non MST as your original question suggests (3 per card), then have to use the two cards separately, and depending on resolution of the projectors you may even daisy-chain some if supported.
And unlike what was mentioned of course Xfire+Eyefinity (even in arrays) has been supported since the old Cat9 drivers, but it's effectiveness is an application to application thing and picking between the possible 10native (12 total) outputs is tricky. Really very app/situation dependent.
Hopefully you can find other help, cause I'm just stopping by (don't bother PM'ing won't check for months). Good Luck!