Before anything, pointing out portrayal difference about them.
1) Ryuo (left part of the main image)
As Luffy actively shown Ryuo and the explanation matches Zoro's Breath of All Things, I'll be expanding on the Sanji part.
The Sanji panel is brought up by a lot of people as simple steam, but the image below explains why that's not true.
There's also a panel in Thriller Bark that can be used as it was foreshadowing of it, and again not coming from his legs but upperbody.
Plus, it would only make sense that a armament boost happens in the place where it was born: Wano.
Now, before expanding on the other part of the image, I'd like to debunk some main arguments that seem to be famous among the community about Sanji, since most agree or assume that Zoro is awakening it at some point (and Luffy obviously has it).
A) Sanji only cares about women
B) Sanji isn't a leader
C) Important times when CoC was displayed, both Zoro and Sanji were at them
C.1) Back when Rayleigh showed it (Usopp was at this one too, who knows maybe a foreshadow)
C.2) At Fishman Island, the interesting part here is that Jimbei and Robin treat it like something big while Zoro and Sanji are quite casual about it.
D) Sanji has a weak will or holds no ambition
Last chapter we saw that Sanji still cares about being a big deal to the marines, so he didn't forget what he said at Thriller Bark.
Speaking of that moment, Oda even makes a parallel between Zoro and Sanji about getting famous.
2) Conqueror's haki (right part of the main image)
After all this prologue, time to explain the other part of the picture.
What we have are characters in different chapters using the same kanji but normally they aren't unique ones, unlike the one in question.
Those are the two powers that connect them three. This doesn't exclude upgrades in Luffy's fruit, Zoro's blades or Sanji's flames or something related to observation haki, but this isn't the point of the thread.
Funny detail, when the summup of Wano is done, they three appear at the back with Kaidou.