Are these big enough to cut 12/3 electrical cable and is there enough torque to do so for a 50 year old with mild arthritis?

Yes however, compound action trades off jaw capacity for increased cutting force and mechanical advantage. This means you’ll have more cutting power at the expense of jaw capacity. So the Romex cable might not fit all the way down to the craw of the jaws but it should still be able to be cut with ease due to the increases cutting power.