Is CD3 the play disk? If so, did you buy the VI expansion? It replaces the MTW play disk as the one that needs to be in your CD drive. So if you still have MTW installed on your computer, you could buy the expansion and never need your original CDs again - assuming you never want to change your computer... (Anyway, by the time you upgrade, MTW will be in the bargain binds as STW is now.)

The VI expansion is worth getting in its own right, so you would not really lose out.

If you have VI or CD3 is not the play disk, I am sorry.