Did you succesfully uninstalled the CS2? If you had any problems during uninstall you probably have issues with your registry.
If you didn't receive any messages when uninstalling, try installing and uninstalling CS2 again.
Download some registry fixing tool and check your registry out. If all of this fails then try reformatting your PC. If you cannot reinstall your OS for some reason, then you can always download a demo version and try it.
If all of this fails, buy some gasoline and burn it all.
You can try installing this CS3 in another PC to see if the problem is yours. If it works, deactivate the license in that PC, uninstall the CS3 and this time you will have to reinstall your OS.