Context over "rank." Zoro has CoC, the highest durability for a regular human, the physical strength to overpower Zoans, and a BLADED weapon that provides range and defensive boosts.
Yamato is a super tough Zoan with unique defensive and offensive abilities, but that's not necessarily more than King. Yamato also has CoC, but has yet to actually finish a strong opponent so we can't rate it. A blunt weapon too, which means less likely to finish on contact.
Sanji has involuntary mutant powers. Decent Haki, great speed, and a lot of skill, but without control of the Germa boost it's not really good enough to beat the other two. Maybe Jinbe if he ever gets careless.