This should work for computers attached via ethernet cables to the router.
Use the "setup a home or small network" wizard. It should be available under the network connections window. Look in the top left hand corner. Run this on the first computer, again as the other poster stated, this is assuming that you are using Windows XP on all of your computers.
When you get into the wizard, Select the option that states "This computer connects through a residential gateway or another computer" You can just leave the name and description the way they are by default, Then when you get to the Workgroup window, you want to write down what it has there, Or you can change it to something else, you will need this for the other two computers. Then turn on file and printer sharing. When you have done this, you can then run the wizard on the other two coomputers, if you need to change work group names you will have to reboot the pc's before the new settings take place. Also the printers SHOULD automatically share between the computers with XP installed, and you will also have the default shared folder on each PC called "Shared Documents". At this point you should be able to browse the network, under the My Network Places group and see the other two computers from the other ones.
As I stated the above steps should work for computers connected with cables. Now if you are doing this via wireless instead, the first thing you want to do is use the Wireless Network setup wizard, and setup the wireless network.
When you run the wizard it will allow you to save the settings to a thumb drive, which makes setting up the other computers painless for the wireless network setup. Now after you have the wireless network setup, go back to the top here, and use the instructions above and you should be using file and print sharing between your computers in a matter of minutes at that point.