I have a 3 gang box with qty 2 single pole switches and qty 1 three way switch. Box is fiberglass and each switch has a grounding screw. The 3 way switch is on a completely different circuit breaker. Other 2 switches are served bay another breaker. Question is should all 3 grounding wires be connected to each other? Also do I need to ground the switches themselves also? Thanks and if it not too much trouble explain the reason(s) so it helps me understand.