The new interpretation of the licensing only applies to systems created after that date. Existing systems are not affected.
Just a point though on a different matter. You have 3 PC's, all running XP, and you want to upgrade all 3 to Windows 7. I am unsure if a single W7 upgrade disk is sufficient to upgrade all 3. If you only purchased a single W7 upgrade disk then you may want to carry out further research on how many times it can be used.