Following a strenuous training session, the muscles could become very sore. In fact, I've put some extremely well conditioned athletes through a challenging workout and seen where they had difficulty walking without pain for several days. If an athlete...or non athlete... were to try to get another workout, for the sake of "getting three workouts per week",despite the lingering soreness, the muscles would more than likely decrease in strength. Recovery is an important slice of the pie! I've seen where athletes actually become chronically sick or prone to injury when overdoing their training. Too much of a good thing is.... well...not a good thing.
Too little recovery time does not allow the muscle tissue ample time to rebuild itself to a higher level of strength. Continuing along this path will lead to overtraining and the problems and health concerns associated with it and may possibly lead to compromising the immune system.
So keep in mind that Rest & Recovery are very important factors in trying to get to The Next Level.