Monster trio is rather moot concept anyways because Luffy Zoro and Sanji were called three monsters of the crew in Alabasta because in comparison to what they had at the time, Nami and Usopp, the three literally were monsters.
That dynamic was quickly broken when Straw hats got another monster in the crew in Nico Robin, yes she was a monster by Alabasta standards and its debatable she was stronger than Sanji in Alabasta.
Beyond that, that concept has hardly been reinforced in manga, unlike Luffy and Zoro's due which has long term story telling, Pirate King and Great Swordsman, Mihawk calling them great team, the symbolism in Thriller Bark, all data books attributing to Luffy and Zoro's closeness but Sanji never getting that shout out, Worst Generation.
Sanji is strong, but always was and always will be one tier below Luffy and Zoro.
Currently there is someone else who is same tier as him in Jinbei.
Luffy and Zoro - Captain and First Mate/Parter, Pirate King and WSM, with larger than life ambitions and comparable will power and desire to get stronger
Jinbei and Sanji - Two very strong fighters by nature but no ambition or exceptional will power to pull them to next level