Mini ramble/theory
Each represents/parallels an ancient weapon
Luffy - king of the seas - Poseidon
Zoro - king of the underworld - pluton
Sanji - king of the skies - Uranus
Maybe each ancient weapon also representing 1 of the 3 haki
Poseidon representing conquerors
Pluton representing armament
Uranus representing observation
Each m3 likely getting conquerors and specializing in it in their own ways
Sanji mastering his conquerors through applying observation haki techniques like idk
Zoro mastering his conquerors through applying armament haki techniques like flow
Luffy mastering his through both and maybe something else
Each represents/parallels an ancient weapon
Luffy - king of the seas - Poseidon
Zoro - king of the underworld - pluton
Sanji - king of the skies - Uranus
Maybe each ancient weapon also representing 1 of the 3 haki
Poseidon representing conquerors
Pluton representing armament
Uranus representing observation
Each m3 likely getting conquerors and specializing in it in their own ways
Sanji mastering his conquerors through applying observation haki techniques like idk
Zoro mastering his conquerors through applying armament haki techniques like flow
Luffy mastering his through both and maybe something else