John Hilley
Well-known member
What you have planned should work fine. You don't need a 30 amp breaker, that is in the pedestal you are plugging into. You don't need a ground bus, just connect the three grounds together with a wire nut and an additional ground pigtail that you connect to a screw in the threaded hole you see in the box.
Ground pigtail
http://www.homedepot.com/p/Ideal-Grounding-Pigtail-12-AWG-Solid-Tail-5-per-Bag-Standard-Package-is-4-Bags-30-3392S/203734801
Ground pigtail
http://www.homedepot.com/p/Ideal-Grounding-Pigtail-12-AWG-Solid-Tail-5-per-Bag-Standard-Package-is-4-Bags-30-3392S/203734801