setting up a network of 3 computers

I have a small office that wants to add a new computer making a total of 3 computers. They want to use / and share excel and word files. Could i set up the new computer as a “server” with all running win xp and in the same wprkgroup? They want to be able to work on the same docs from all the computers but have it lock so only one person could be in the doc at once. Does this happen automatically or would I need to do something special?
thanks
Jan