If you are just going to do file and printer sharing, doing it in Windows is much easier than on Linux. However, you might be able to set up a Windows Domain using Linux (which you cannot do, as far as I know, on Windows XP- you'd need Windows Server, meaning $$$). The advantages of a Windows Domain on a 3-machine network are, on a practical level, probably not worth it, but the tinkering would be interesting.
edit: That of course if your laptops are running XP Pro. If they are running XP Home they won't be able to join a domain.
As for the things you can do on a Linux box, that mostly depends on your interests.
I you have system administration inclinations, I would definitely recommend you to install Linux, learn to set up Samba for the filesharing/printsharing, remote administration via SSH, etc.; basic Linux administration skills are interesting even for Windows-focused admins. You can add more daemons you might want to learn to admin, such as a web server (Apache's httpd), mail server (Postfix for SMTP, Dovecot for IMAP), db server (PostgreSQL), etc.
On a higher level, there are tons of apps which run pretty well on Apache+PHP+MySQL which you might want to know how to install and admin, such as, for instance, Mediawiki, Wordpress, etc.
If you are more development-bent, you might want to learn how to set up a server for your apps and deploy something to it, such as a PHP app, Java app, etc. This is interesting, as it's fairly cheap to rent a Linux virtual server, and having one at home is excellent for getting acquainted. Then you can deploy apps you develop on the shared server so that they run in a "pro" environment (network redundancy, UPS, etc.).
You can also run anything you want to leave running 24/7, such as downloads, or want to have available from everywhere (people still run IRC clients on Linux boxes so that they can access them from everywhere, for instance).
Nice point about the Windows Domain. – Nathaniel – 2010-01-18T05:25:57.893
XP Pro can handle domains, IIRC, but not XP Home. Not an expert, could be wrong. – CarlF – 2010-01-18T05:50:20.317
thanks guys, edited the post to reflect that. Creating a Windows domain will work out really well if his laptops are running XP Home :-p – alex – 2010-01-18T20:21:38.910