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.

You really have no choice but to run more romex. The 12/3 coming from that light has no neutral wire. It is two travelers for the switches and either the hot feeding the switch or the line out to power the light.

Whether of not you have a neutral at the 3-way switch depends on how it is wired. If one switch is being fed and the other has the switch leg to the fixture you will have a neutral at each switch. If the switch is being fed and also has the switch leg to the fixture, you will not have a neutral at the other switch. If the switch is being fed and the travelers pass through the fixture box, you will have a neutral only at the switch being fed. If the fixture is being fed, you will not have a neutral.

the end of the run 12-3 wire coming from my upstais 3 way light is currently attatched to a single switch.

The words "end of the run 12-3" say to me that the only thing in the switch box is a 12/3, in which case there could be no neutral. Though I suppose it's possible that what matt wrote was not accurate. Unless matt2008 tells us there are more wires in the box I stand by what I wrote.