I just had to do this on two Intel iMacs in our Art Studio.
I deathorised CS2 first, to avoid any potential serial number issues, even though I was using an official CS3 upgrade license.
Take your time and uninstall CS2, following the directions as closely as you can. Adobe ask you to MANUALLY trash stuff, even AFTER you run the uninstallers and not everything will be where they say it is.
When installing CS3, be patient, as the installer progress bars update so slowly it's easy to think the installation has stalled or hung. This is even MORE prevalent when installing the roughly 450-550Mb of updates that the Adobe Updater will want to download after first run. These appeared to hang for 15 minutes(!) but thankfully I left it to finish, even though it appeared utterly hung even reporting "Application not responding" in Activity monitor.