Luffy = Godlike Mythical Zoan Devil Fruit since episode 1
Zoro = Gets awesome swords "cursed" swords with a life on its own, including a Black Blade from an actual legendary swordsman. Basically free power ups and training tools
Sanji = Trained by Red Leg Zeff, and was often was found in the need of beating pirates who tried to attack the Baratie. Got his first power up in Enies Lobby, which doesn't comes from Germa, even if Sanji has complete control of it, currently.
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Luffy = Trained by Rayleigh, the Dark King, known and respected around the world, his name speaks for itself --> Learned CoA, CoO and bettered his CoC control
Zoro = Trained by Mihawk, known as the Strongest Swordsman alive in the world (and whatever that implies) --> Also learned CoA and CoO
Sanji = Beat some Okama, ran from them like crazy to the point he learned CP's technique (Geppo) solely by running, created a new one that works in deepwater (Blue Walk), got some revigorating recipes from a Rev Commander to support crew with his cooking --> Also learned CoA and CoO
Recently:
Luffy = Learned Ryuo, CoC coating (CoA 3.0 at this point), plus Gear 5 God Mode
Zoro = Got Oden's sword, trained with it, and unleashed its potential alongside his own potential. Used a Mink Elixir to be able to fight again. Learned CoC coating
Sanji = Almost died 234th times and made choices that put his life, his principles and his pride aside since his childhood so then he could eventually awaken a "Veteran Level" Exoskeleton, the same from which his siblings get a huge part of their strength, and even then still depend on their Raid Suits, while Sanji destroyed his.
Keep in mind Sanji has 21 years old, and still has a lot to achieve. Mind blowing, isn't it?
Thoughts?