If wired, connect a router WAN port to the Internet and then connect each of the 3 computers by cables to the LAN ports on the router. Make sure that the router DHCP function is enabled.
If wireless, connect the router WAN port to the Internet and then make a wireless connection to each machine -- they will each need wireless cards for this. You need to enable the DHCP and wireless capabilities of the router. HERE is how you find and enter your wireless security password.