1] Look at your firewalls. They all need to know the network connections of the others.
2] Each XP machine must have accounts for the other two, the same as those other two. EG, if computer 1 has an account name Bloggs, then the other two must also have accounts for Bloggs. And so on.
Mark
I have XP on three PCs. I have a deskotp hooked to a dsl modem/router. I also have two wireless notebooks connecting to the same modem/router. The problem I'm having is I can't access the shared folders on the two wireless PCs. I can, however, access the desktop from the wireless PCs. IS there any reason behind this?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic