1) All editions of Office 2007 (other than the Home & Student) can be installed simultaneously on 2 computers only- a desktop and the owner's laptop. Home & Student is good for 3 simultaneous installs. So check what edition you have, since the package says 3 PCs, you probably have the Home & Student.
2) What probably happens is that activation info is discarded by MS servers after a period of time (only MS knows how much exactly), hence letting you install on more than the allowed number. But be aware that you are violating the EULA by doing so. It is also possible that the violation may be detected by MS, so you could end up with Office deactivated on one or more machines.