Technically speaking Sanji did take down Big Mom, just not in a fight.
Anyway, that's not really my point. My point is that Oda drawing characters together isn't really much of a sign of, well, anything. Well, maybe that's not fair, he hasn't drawn them together for no reason. But I don't really think it's a sign that these three are going to fight Kaido and no one else will. In Wano, he's drawn Luffy, Kid and Law together. He's drawn Luffy, Zoro and Kaido together. He's drawn Luffy and Zoro together. He's drawn Zoro and Sanji together. In this chapter we also got a brief Luffy, Zoro and Sanji bit as well, I think we've even had a Luffy, Law, Kine'mon, Inuarashi panel collection at one point, though that might have been Zou.
End of the day, though, I'm still not expecting anyone other than Luffy to do more than be side players against Kaido. Land a hit on him, maybe. Get wasted by him to hype Luffy vs Kaido, definitely.