I have not done that much in that area. Maybe someone here knows more than I. Here are some of my observations.
I am assuming that you have all of the computers on a network. If not, that would probably be the first thing to do.
You would think that you could just map the drives that you want to back up on the other computers to the 4th computer and then use any backup program to back the mapped drives. But computers do not always share nicely over a network.
Maybe if you could find software that does this, it would have step-by-step instructions or if you can find someone who has done that.
A different option might be to get a networkable drive. Here is an article on that - http://www.pcworld.com/article/121967-2/backup_over_a_network.html