let me explain
suppose you are training any muscle,but for example lets take those 3,chest,biceps and triceps.lets assume that you are advanced and doing 12 sets for chest.you start your first chest exercise but you feel weak,you can't handle the poundages that you usualy do,if this is the case,don't do your full chest workout do anywhere from 6 to 8 sets only,reserve your energy for your other muscles[biceps,triceps]and go and have a better stronger workout with those.you will be surprized,a bad day for one muscle could be a good day for the other.trust me,it happend with me more than once.