OK, to diagnose this problem we need more info!
1: What type of monitors? (Model, Native resolution, Available inputs)
2: What type of DP-DVI adapter? (is it active? active ones are bulky and need USB power)
3: What graphics card?
4: Are you trying to use Eyefinity? (ATi Radeon HD 5 series only)
If you're trying for Eyefinity, all 3 monitors should be the same resolution (and size, but that technically doesn't matter, it just looks better with them all the same size). You NEED a Radeon HD 5 series video card and one monitor must use DisplayPort. In fact, it must use DisplayPort signals, so a passive adapter (which uses DVI signals over the DP connector) will not work. This means that 2 monitors connect via DVI (or HDMI/VGA) and the third monitor must either use a DisplayPort connection (if the monitor supports it) or an ACTIVE DP adapter (these exist for DP-VGA and DP-DVI but the DVI ones are over $100 and require USB power). If your monitors are 1920x1200 or less, you can get by with the DP-VGA adapter and use a VGA connection on the third monitor, it should look fine (this is my setup, 3 Dell st2210 1920x1080 monitors, 2 on DVI, the third is on VGA through a $20 Sapphire DP-VGA adapter and all 3 work together well on my Radeon HD 5870).