My thought is worst-case, they could sit on a lap to reach. I think they'd have fun just with the excitement of the lights and noise and all the action around them. If they're pulling the trigger on the gun and aiming at bad guys, they're going to have fun with it - they really won't care if they're hitting targets or scoring.
But I have to admit, we have no first-hand experience on Buzz with a 3 year-old.