If you have already installed 3.0 and nothing bad happened, you just might be ok. I would still take a free registry cleaner like Eusings Free Registry Cleaner (just google it) and scan your system with it. Also, CCleaners registry cleaning system might help out, though some have said it is too aggressive I have never had a problem with it.
Evil99019, I usually just scan using Eusings and let it erase all the registry cleaners it finds that need to be cleaned. I find it pretty conservative in its scanning and it has never done a bit of harm to the registry. Another one you can use that is highly recommeneded is reg seeker, though I have never personally used it. Still, I would make a backup of it before cleaning, Eusing does this to, go to file, backup full registry and save it to a spot on your PC. If you don’t want to do a complete backup (highly recommended) export the registry key before you delete it this way you can replace it if necessary.
I would be very cautious about uninstalling version 3 and deleting registry keys manually as one false move can seriously cripple a PC or even force one to format an operating system completely. Having said that, if you feel confident there is some talk on the forums about deleting individual registry keys. For some reason I can’t find them right now, but if I do I will post back here and let you know. You could also try to search for them somewhere.
Edit Hahaha, nice job USSS. USSS found the thread I was talking about as I was trying to find it. Please disregard my statement about trying to find it. You should probably try a free cleaner like suggested, overall I think it is safer. To do it manually involves the risk of getting “trigger” happy an uninstalling anything with the name Comodo…