It isnt about how long you lift. Anything over an hour for most people is overkill. Cortisol, the muscle wasting horomone usually begins to rise around the one hour mark. My workouts only last half hour, and sometimes even less. IMO, Intensity is the key to strength and hypertrophy. While you will get differing opinions on growth, most will agree that the quicker you get in and out of the gym, the better.
"Show me a beautiful woman, and I'll show you a guy tired of screwing her"
This all depends on the intensity of your workout... Also depends on your aims, what are you 'aiming' for, Gaining Mass, Cutting...Extreme Bulking ? all depends.
If you use a good split up routine which hits each part of the body within those 3 days and the intensity is high and you have a good balanced diet, then yea...sure you should see gains(Of whatever you are looking for) Just make sure its intense and you dont sit around 10minutes of that strict 20 minute timeslot of yours.