Which make and model of laptop? If it has an 10/100 ethernet port for an RJ-45 cable you can do it using cable.
If one of the computers has a wireless card installed you can make an ad hoc connection. If not You will probably be able to do it by putting a USB wireless adapter into one of the PC`s (Cost about £25) but again it`s ad hoc.
What OS are on the other two computers? Wait for more advice anyway. click here for background reading on ad hoc networking.
click here for a brilliant simple guide for all systems.