If it's on the same machine, you probably wouldn't use up one of the other two copies if you reinstall. If you do, however, I wouldn't worry about it. If you can't authorize the third copy when you go to do so, just call M$ (the program will tell you who to call) and tell them you have Office installed on only two other computers. They may need to deauthorize the existing installations but it's no big deal to reauthorize them. I've done this a couple of times with my copy of Office 2007; it's been installed on five different computers although no more than two at one time. I gave it to a friend when I got Office 2010 for use on her son's computer (it's the same version her son's school uses) and he had no trouble installing it.