To calculate how much heat every 6990 in the setup will produce, you have to consider how much energy the computers containing them will draw from the wall. PSUs aren't 100% efficient, and all the "lost" energy gets converted into heat as well.
Calculating 65 W power consumption outside the video cards, we get a total of 1565 W for a computer holding four 6990s. If the PSU operates at 85% efficiency, the whole setup will draw approximately 1840 W from the wall, giving 460 W per video card.
It is far simpler. Computer components (including GPUs) do no "work" in the physics sense. So 100% of energy drawn from the wall will be converted to heat.
If you want a 3KW heat just build a rig (or rigs) which have a 3KW load. No other calculations are required. It doesn't matter where the heat come from (CPU idling, GPU core, VRMs, power supply ineffciency) it is all heat.
3KW electricity In = 3KW thermal energy ("heat") out.