Hi Sara,
3 day workout routine is specifically designed to get the benefits of exercises and to save time. So the biggest benefit of 3 day workout routine is for those who have short of time for exercise due to busy schedule. Usually in 3 day workout routine your workout is split so that you give apple time for particular groups of muscle to exercise usually 2-3 muscle group is focused each day. Along with this your body gets sufficient recover time after the workout as muscle grows during the rest period. 3 day workout routine is very effective especially in case of strength training as risk of injury due to training is minimal.