So you need:[*]rear derailler hanger[*]rear derailler[*]front derailler
The rear derailler hanger should't be that difficult to find, if it's within the past 10 years, and since it comes with a triple crank and 9-speed sprockets, I'm assuming that the bike is from within the last 10 years. You may have to contact Cannondale to find one, or just go to a bike shop that's a Cannondale dealer and they should be able to get one.
Getting the right deraillers will depend on whether you have Shimano components. If your brake-shift levers have a cable popping out of the inner side and then curving in towards the frame, you have Shimano. If your brake shifters have both the brake cables and derailler cables routed under the handlebar tape, then you have Campagnolo components.
Assuming that you have Shimano components, you need a long-cage rear derailler and a triple-compatible front derailler. A triple crank means a wider ranger of gears, which means a wider range of chain slack that the rear derailler will need to take up, so you need a long-cage rear derailler. A triple-compatible front derailler will have a longer cage as well, to be able to handle the wider range of gearing.
If you want to go cheap, you should go with Shimano 105 or Tiagra components. They will work flawlessly, but be a touch heavier or not as nicely polished as Ultegra.
Remember: triple-compatible rear and front derailler. Good luck.