Solved
Cannot start game, Another instance of the game is already running
After I relocated the downloaded game location from orgin (along with the installed files), my ME3 cannot start. Every time it displays 'Another instance of the game is already running' error.
Do a full un-/re-install (make a copy of your saves folder first!)
You can specify where you want your games installed when using EA/Origin by going to the "Application Settings -> Advanced" tab.
Won't take long, and I'd bet that the problem is a registry entry that's pointing to the wrong location.
