Just like Rayleigh said:
"unlike the other types of Haki, CoC cannot be attained through training, instead its users must be born with the ability and can only learn to control it after discovering that they have it".
Luffy specialized in CoC coz that shit is based on his will. Luffy will >>...
This is my personal thought about their power ups:
Luffy - Comes from powerful lineage. He got a df and it gives him few advantages over others but we see how he train hard to master it alongside his haki. Genes + training.
Zoro - Comes from powerful lineage too. He trained the hardest among...
List of power ups for Monster Trio
Luffy: Advance CoA, advance CoC, hopefully gear 5 is incoming
Zoro: Advance CoA, CoC, Enma, black blade
Sanji: Raid Suit + Germa body and maybe advance CoO
p/s: Can't wait for Raizo power up
THE GREATEST SHINOBI !!!!!