edited:
If you buy a pack which includes licences for three PCs, they all expire at the same time, taken from the first install date. this should be in the documentation/terms & conditions.
Next time, a useful trick is to buy a (genuine) pack off Fleabay, from a dealer who has an overstock to shift.
When they bring out e.g. Norton 2009, all the Norton 2008 packs look obsolete and need to be shifted cheap; but they still give you 12 months of free updates, which for the Antivirus file is a good bargain. You might even be able to get an old copy of 2007 even cheaper. it is essential to get a genuine pack with an unused Product Key.