12/3 wire feeding additional light in dinning room.
The end of the run 12-3 wire coming from my upstais 3 way light is currently attatched to a single switch. I want to install a double gang box at that switch and feed power to 12/2 wire, through the switch that will power my dinning room light, that will run up to my dinning room light. In theroy I want to add power to my dinning room light from the end of the 3 way switch to the switch that will control my dinning room light. Or should I just run new wire from a junction box connected to the circiut controlling my downstairs into the 2 gang box and if so can I connect the ground in that gang box together even though they are on different cicuits?? Sorry to make it so difficult.