How hard is it to link three vaio computer and make one the central hub. I want create a work station for my students but have the ability to check their work from the main computer

Quite straightforward.
You will need to physically connect them all and assign them all IP's in the 192.168.0.x range. A switch and some ethernet cable will do the trick.
Then share a folder on the "hub" machine to all the others.

I'm not sure what you mean here

have the ability to check their work from the main computer