Hi Nancy,
3 day workout plan is specifically designed to get the benefits of exercises especially for those who have short of time for exercise due to busy schedule. Usually in 3 day workout routine your workout is split into 3 days so that you get 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.