Indeed Form III is a potent form which is why it is my premier form however Form II requires great skill.
However you are incorrect as to your definition of Form III. Form III is not a stance but a way of fighting. I will elaborate:
Form III aka Soresu is a defencive form where one is not the aggressor. He lets his opponent attack and counters it by exploiting the opponent's openings. Yes it can be best done in yellow stance but it can be done in the other 2 stances as well. If you need a detailed explanation on the forms let me know.