First off, you'll want to make sure that your copies of XP and the SP3 update are duplicated. Don't have just one copy. If your only copy of SP3 is on a flash drive, you're done when (not if) that flash drive dies unless you're lucky enough to find another copy elsewhere. Back it up. As well, if you have a retail or OEM copy of XP other than from such as a brand name builder, you can slipstream your copy of SP3 into your copy XP disk. Slipstreaming creates a new bootable copy of the OS on another disk so you don't need to do the upgrade after an installation.
If I reinstall Windows after July 2010 but I only have an XP SP2 Upgrade CD. Can I just installed Sp2 normally and then install Sp3 from a saved file on my USB?