I think your watercooled HD6970 Tri-fire is very comfortable with 3x1080p overall speaking.
For 3x27'' (3x 2560x1440), use the table below for estimation:- (note the driver used was not too recent)
1) For the six columns (4 through 9) involving HD6970s, only column 8 is Tri-Fire while the others are 2-card CF. Unfortunately, the Tri-Fire 6970 setup gives little info as it crashed so often (likely due to using Portrait Mode which is not much supported by earlier drivers).
2) I suggest you to use Landscape Mode (vs Portrait Mode) as it is more 'stable'. Therefore, use column 6 (in red rectangle) for main reference. Going from CF to Tri-Fire, the theoretical max is 1.5. So, say, apply a 35% (30% for GPU scaling and 5% to account for 2560x1600 vs 2560x1440) gain in those ave framerates and then see if the fps are acceptable you.
Also, member Levesque
is running 3x2560x1600 and I think he has some experience with HD6970s at this resolution as well. PM him for advice.