A lifting friend asked me the other day: "You say that doing three exercises per muscle group is better than doing five. How is this possible?" Traditionally, I would answer this question by explaining hypertrophy and how once you are past a certain point lifts become futile and can even have negative effects. But before I answered his question I reconsidered. I took something for granted that I think most people are not aware of. A lot of people that lift, lift a lot of different exercizes (5,6,7+) per muscle group per lift for one reason: They aren't lifting hard enough.
I turned to my friend and asked him, before I answered, why he does five exercizes for his chest instead of three. He told me that he does five, because unless he does five he doesn't feel a burn in his chest and he doesn't feel muscle fatigue the next day. Ah-ha! Exactly what I suspected. He was going through his routine without maximizing the weights/reps/volume or any combination. He was lifting but not to his full potential. I suggested that for the rest of his workout he bump up the weights and add a rep to each set and only do three exercizes instead of 5. We were working chest.
The next day at the gym I saw him and he looked like he could barely lift his arms. He said, "holy **** does that ever make a difference. My shoulders, chest, and triceps are on fire today. I am just going to do cardio." I chuckled and told him to definitely rest well during the first stages of a newly adopted routine.
So in summation, two words to all, LIFT HARD. If you think you are lifting as hard as you can, you aren't most likely. Get a spotter and USE him. Especially on your last set you should come damn close to, if not needing, help on your last rep or 2.