I think you should start off by change your workout and workout schedule. Instead of 3hr workouts, keepm it at 60-75min. Your body will appreciate less workout stress
and therefore will have time to help your muscles to recover and build stronger and larger muscle tissue. All the Big and Strong guys know that rest is key to growth and recovery. Concerning your workout schedule, Its time to follow a Split Routine in which DAY1 is chest, back, tricep and DAY2 is
legs, shoulders, bicep. If I read correctly, you do "3 hour workouts for all the body parts", HERE ME OUT! THAT WAY TOO MUCH TRAINING TO ALLOW FOR CONTINUED GROWTH AND DEVELOPMENT. You can also try a 3-DAY Split Routine such as, DAY1 chest,back DAY2 shoulders,legs and DAY3 bicep, tricep, calves, abs. On your days off do some cardio and abs to get rid of your belly.