Linux Data Server

Right now I currently have 3 laptops
-one asus g72 (gaming)
-a gatewaye (hosting game servers)
-acer (going to no use)

well since I have three laptops, two of which are in use, i wanted to make the acer into a data server so I would be able to backup my data from the other two laptops.

How would I go about doing this?
All three laptops are connected to the same network so they all have internet access.
The Acer is currently running the latest version of Ubuntu and has nothing on it.
I would like to make it so that the whole computer is one storage device that is connected to the other two laptops.
I’m currently confused about how i could do this, so a step-by-step process would be great.