What you are describing is a serverless "Peer to Peer" network (if you want to read more).
Where on your XP computer, you setup a directory share.
It really works well if you have the same login/password on all the computers - as you will be automatically logged into the other computers (the "share" won't continually ask for logins).
Dropbox is another option (
www.dropbox.com)
You essentially setup dropbox on all 3 computers and it will handle the syncing.
It is free for sharing under 2 Gigs.
It sync surprisingly fast.
The other benefit of this ... is (1) the files are ONLINE = a remote backup, (2) the files are on ALL the computers (3 copies is safer than 1), and (3) you can take the laptop, work on the files offline, then when your laptop connects to your WIFI, all the syncing will be done .. automatically !
Dropbox also, FOR FREE, versions your files .
Dropbox is also easy to deploy. I say get that going first ... and see if that meets your needs