personally I'd sell 1 and keep the other 2. The thrid card for sli is going to have tiny gains over 2 and you'll mget well over 400$ if you sell it.
more money, similar peformance.
or see how much 3x 6970 would be. liekly the 3x 6970 would best the 3x 580's and 2x 6990's due to how well they scale compared to nvidia and simply how poor the 6990 performed compared to the single gpu variant.
results at 5:15, note the minimum frames. not quete the same but gives you an idea.