You should not buy both since it definitely is NOT a good idea to try an run two different interactive security programs on the same computer.
I know you may think we are biassed here (well we are but it's well based!) but really Norton is better than McAfee on a number of grounds so I defintiely would suggest you stick with Norton which normally comes as good to install on 3 PCs as mentioned.
If you watch the weekend flyers you can find it on sale (eg I see $40 for the 3 PC version) and you can find it cheaper at reputable online sources but be VERY careful since there are scammers about selling used versions or invalid keys. Be careful that you do get a 3 PC version since sometimes in the advertising it is not clear and my first link when I looked just now was for $30 but that was a 1 PC version from a store.
If you do shop online from that well known source I would recommend only to buy where they show that it ships from their own warehouses since I've had a bad experience when they passed the order on to a third party.
You don't actually say which Norton product you have -- Norton Internet Security, Norton AntiVIrus or Norton 360 -- so please tell us. On balance I would recommend the first, NIS.
Also please tell us what versions of Windows you have on each computer -- XP VISTA Windows 7 and what Service Packs have been installed. You can see this if you have a keyboard with the WINKEY (has the Microsoft flag on it) and press the WINKEY and Pause keys at the same time which leads you to System Information.