1) Are you sure you can install it on 3 computers?
If you are referring to MS Office Home and Student 2007 edition, that can be installed on up
to three computers.
Retail editions allow installation on one computer and the owner's laptop.
OEM edition is tied to a single computer, moreover the license cannot be transferred to another computer.
To view the MS Software License Terms for your product after it is
installed, do one of the following in these 2007 Microsoft Office system
programs:
Access, Excel, PowerPoint, or Word
Click the Microsoft Office Button, and then click Program Name Options,
where Program Name is the name of the program you are in, for example, Word
Options.
Click Resources, and then click About.
Click View the Microsoft Software License Terms.
2) Non-OEM editions can be transferred to another computer by first uninstalling on the original computer and then reinstalling on the new one. Depending on how long ago the original install was activated, you may have to do a phone activation. AFAIK, you may reassign the license to a different device any number of times, but not more than one time every 90 days.