"can some one recommend a good gpu that can run three monitors for under 400"
This depends on what you are doing with the 3 monitors - if you are gaming across all 3 monitors you will need
a powerful gpu. If you are only displaying documents & spreadsheets etc. then less gpu power is needed.
I have 3 monitors connected to my GTX 970 a Dell 30" 2560x1600 and 2 Dell 24" 1920x1200. The 30' takes a DVI-D, one 24" has a display port to DVI adapter, and the other Dell plugs directly into the DVI connector. What type interfaces do your monitors have? Look for gpus that have the proper connectors for you - many will run 3 monitors if you're not gaming on all 3.
-Bruce