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-internet-connection-sharing#1TC=windows-7"