This pattern of three strong commanders is non ecxist. Big Mom even has Katakuri, Cracker, Smothie and Perospero who is Jack level. Whitebeard had Marco, Vista, Jozu and Ace, and we can cite Izou as an opponent of Jack's height, Ace's bounty was low because he was young. All nine of Shanks and Teach's commanders must flirt between levels of YC3 and YC1, with Ben Becham and Shiryu at the level of an admiral. Luffy can have many commanders too: Zoro, Sanji, Yamato, Jinbei, Robin, Franky, Bartolomeo, Sai, and Cavendish. There's no problem.
It's not hard to argue that Robin is Tier YC3, as she easily defeated someone with nearly 500 million bounty, and Hakuba who had 330 million in Dressrosa. When Robin decided to use his true power, it was Low Diffs in Black Maria. I can see Robin beating Perospero without too much trouble.