If you want the same exact copy of your iTunes Library (playlist, ratings, songs, etc.) on both computers, copy the whole iTunes Library from one computer to the other. No need to go into individual song files. If you have protected files, i.e. purchased from iTunes Store, then you need to authorize the other computer. IIRC, limit is 5 not 3.

If your desktop and laptop are Macs, rather than using a portable drive, you can use the "target disk" mode to copy the iTunes Music folder from one computer to another.

Caveat: All my comments above pertain to Macs where all the relevant files as far as iTunes is concerned are contained in one folder. I don't know that this is true in Windows, given the complexities of dealing with DLLs and stuff.