By far the best way is to have all the company data on a "file server" on one machine. You can then back it up daily. You would need to ascertain that one of the machines is suitably powerful enough for this or have a fourth dedicated machine.
If you are stuck with the three machines you will need the "owners" to leave them on for access from other machines but not necessarily logged onto the network. This could raise security and privacy issues and much depends on your Operating System. XP being more versatile and user friendly when setting up sharing options.