For AMD, I'd recommend R9 285 or higher. I have run that particular card on 3520x1080, & it was bit sluggish. For you it would be good (but low-ish life-cycle I think). The R9 380X is very similar, but bit stronger (& overclocks bit better than R9 285). So this is probably your best match. If you overclock it, you'll get a good life-span. These solutions are for the money-minded person, wanting pretty-good card that can't always max games.

But to max heavy games, you will want to go bit higher. The R9 390 would have you covered (the next step up I think), but it uses GCN 1.1, which is older tech than R9 285 / R9380 (which use GCN 1.2). So some features on R9 390 are limited (e.g. no Eyefinity PLP support). I would recommend AMD because they are leading tech with new features, but I have no real allegiance.

Shit sorry I misread your post. I missed the whole "triple" part. I will answer proper later if no-one else does.
EDIT:
Card Recommendations:
You have 2560x1080x3 = 8,294,400 total gaming px.
= 2 high-end graphics cards will run it decent (somewhat short life). E.g. R9 390 or 390X.
= 3 high-end cards will run it well. E.g. R9 390 or 390X.
= 2 extreme cards run it very well, bit overkill. E.g. R9 Nano, R9 Fury or FuryX.
= 1 SUPER-extreme card runs it very well, bit overkill. E.g. Radeon Pro Duo (releasing 2016 Q2, it is equal to two "extreme" graphics cards)

This all is rough but close.

Unrequested advice (ignore bias as needed):
I find it a bit disturbing that people are combining ultrawides for MM. Think what your games will look like.
If you have not yet purchased your monitors, consider your aspect's width VS your height. Is triple Ultrawide truly what you want? Don't let me dissuade you, but think about it. Your chosen aspect will cause many games to look not great, due to side-stretch FOV anomaly (extreme fish-eye). Your vertical vision also will not be covered, while your setup's width is actually wider than your vision (wasting PC resources). If your monitors can cover your vision (like glasses), it is the most immersive MM experience. All that said, some people are doing triple Ultrawide, to each his own. I am not into personally it.

Just because I know AMD better & have not researched Nvidia cards recently. So AMD suggestions give you the approximate idea, but use what you want. Look for similar strengthed Nvidia cards. BTW specs for the AMD cards recommended are here:
https://en.wikipedia.org/wiki/AMD_Radeon_Rx_300_series