If your 3g modem has a eth port just plug it onto the switch or router. If it has wifi you will have to set up the router to work as an extender (if possible) to connect the pcs over wifi. If on the other hand it is a usb stick it will tak a bit of setting up...
In case it's a USB stick modem, and your router doesn't support 3G modems, you have two choices: buy a router that supports 3G models (such as http://www.tp-link.com/en/products/?categoryid=202) or enable internet sharing on your PC or Mac (which has to be enabled all of the time). Which one do you prefer?
What you could do is to place the 3G Modem into one of your PC's, then enable ICS (Internet Connection Sharing), Use an network cable to connect that PC with the switch, and then connect the rest of the PC's to that switch.
If you need more help on enabling ICS use this link "http://windows.microsoft.com/en-gb/windows/using-intern..."