FCP3? Really? What computer is this? Or do you mean FCS3?
There are a couple of ways to do this. Either make sure the sequence is unrendered and then export to QuickTime Conversion and use ProRes 4444 or the Animation codec with colors set to Millions+. Or if you've rendered export to QuickTime Movie. In the popup that says use current settings, select ProRes 4444, or at the bottom of the list select custom and make it Animation Millions+.
All the best,
Class on Demand DVDs "Complete Training for FCP7," "Basic Training for FCS" and "Final Cut Express Made Easy"
Author: "Final Cut Pro 5 Editing Essentials" and "Final Cut Express 4 Editing Workshop"