Three monitors @ 2560 x 1080 = 8,294,400 pixels.
To put it into perspective, that is the precise amount of pixels that's in a 4K display (1080p resolution x 4 = literal 4K resolution).
What I don't know is the refresh or these monitors and what types of games you occasionally plan on playing. But generally you'll want at least one GTX 980 Ti. If your main focus was gaming at high refresh with 2015 titles, I'd recommend two 980 Ti's, but that's obviously beyond your budget.
A new 980 Ti is going to be $650. If that $600 figure is a hard budget then purchase an EVGA b-stock (factory refurbished) GTX 980 Ti (
http://www.evga.com/products/ProductList.aspx?type=8&family=GeForce+900+Series+Family&chipset=GTX+980+Ti ).
Both of the cards are currently out of stock. However, if you create an EVGA account now, click "auto-notify" and are prepared to purchase the card at a moment's notice, then I'm confident that you'll have one ordered within the next 5-10 days. That's what I'd recommend. Please be aware the the b-stock cards do not come with Steam codes for a free game.
Addendum:
My recommendation (above) is based on the assumption that you'll be gaming on all three monitors. If you only plan on gaming on one of them then a GTX 980 (b-stock) is a perfect fit. Matter-of-fact, the 2560 x 1080 resolution is the only scenario that I recommend that GTX 980 (non Ti). It would be perfect fit to play 60 FPS on a single ultra-wide 1080p monitor, and not require the enabling of v-sync on a 60 Hz, non free-sync/non g-sync monitor.