If you have two separate switches both connected to the 3g router, then communication between the two switches will be limited to whatever speed the LAN ports are on the router.
If you use one gigabit switch and attach all devices to that switch, then LAN communication will be at gigabit speeds and only connections to the internet will actually go through the router.
If you need multiple switches for the extra ports, then connect the second (and third, etc) switches to ports on the main gigabit switch and devices attached to any of the switches will still communicate at gigabit speeds.