|Do you have a 4th jack in the room? I would assume not.|
You would buy a crossover cable and a small switch.
You would connect the crossover to one of the wall jacks and then to the switch.
You would connect two pcs, one that was connected and one you are adding, to the switch.
Assuming there is a router on the other side of those jacks and its also doing dhcp so all the pcs get ip addresses then you would have your 4 pcs on the internet.
The assumptions are blanks you need to fill in by asking questions of those providing the services.