I've run into this myself before, and here's the answer I've come up with for now:
- 20" LCD running from USB Video card, using VGA cable, running at 1600x1200 (16-bit)
- 20" LCD running from laptop external VGA port, using VGA cable, running at 1600x1200 (32-bit)
For the third display, I can either use the 1680x1050 LCD that's part of the laptop, or I can plug the laptop into a docking station and use a DVI cable to power another 20" LCD.
The USB video adapter is fine for non-design tasks, but I wouldn't trust it for color-critical tasks. I don't know the brand/model off the top of my head, but I'll add it later. It was only $100 or so.