Assuming you are NOT GAMING, then your best bet is fairly inexpensive cards that support three or four monitors.
*The RESOLUTION of the monitor is also important. It doesn't matter whether it's 27" or 10" but is it 1920x1080 or 2560x1440?
So basically you'll want two or three cards. This won't allow them to act as a single monitor (you don't want that anyway) but you can support different applications on each screen and the resolutions can be different.
Links:
http://www.techradar.com/news/computing-components/peripherals/the-complete-guide-to-multiple-monitors-1074313
http://www.amd.com/us/products/technologies/amd-eyefinity-technology/Pages/eyefinity.aspx
*I'm not sure what to recommend exactly. You could get a couple HD6450's for $30 per and get at least two screens each so I think you need to investigate a little further. Sure you can find graphics cards that support up to SIX monitors (with Display Port monitors or adapters) but they're expensive and pointless if you can do the same thing with three, $30 graphics cards.