There should be a phone number to contact Microsoft then. The conversation should go something like:
You: It says I've got this installed on 3 computers already.
Them: Do you?
You: No
Them: Ok, here is what to do.
If it doesn't go that way, then you'll have to explain to them one of the computers you uninstalled it on or the computer died or something. I had to do this with a version of Windows, and all they seemed to care about is whether or not I had it on more than 1 computer. I said I didn't, they gave me the activation code, I'm sure a similar thing exists for Office.