The best triple monitor video card right now is the ATI HD5750. The third monitor MUST have a DisplayPort jack (like one of the newer Dell monitors) in order to run triple monitors.
The best configuration is with one DisplayPort monitor in the middle, with two DVI monitors on the sides. All three monitors need to be the same resolution for best results. The HD5750 can run the three monitors in Extended Desktop mode (display different things on each screen), Clone mode (display the same thing on each screen), or in Eyefinity mode for games that support it (the game screen is spanned over all three monitors).
The price is about $140, so will fit your budget
Go to this page for more info:
http://www.amd.com/us/products/desktop/graphics/ati-radeon-hd-5000/hd-5750/Pages/ati-radeon-hd-5750-overview.aspx