It's not a written rule. By written rules, since Zoro is the closest in power to Luffy among the crew, he should be a bipolar bastard. There also wasn't such a thing in Jojo, and some other manga I could think of with a bit more time.
Aaah but the three strongest in Arlong's crew. They really didnt have any heirarchy among them
That doesn't mean they were equal, either. Just because one would be in Arlong's top 7 doesn't mean they would stand a chance against Nami, etc.
And Bon could hurt Daz if he really got into it. Daz was immune to being cut until zoro came along, but not immune to being hit real hard. Blunt force trauma ftw.
That wasn't stated, though. And by the time Bon could generate enough force, if it was possible, Daz would carve him up. Unless you think Bon's hits are far faster and harder than Zoro's sword strikes.
Sanji still did most of the work in the DBF
My point was that Zoro was limited. In a fight, he'd handle those three easier than Sanji would. Unless you think Sanji has something equal to the 108 Pound Cannon, and decided never to show it.
EDIT: I kind of agree, Octogon. I had them even at the beginning, then gave Zoro the edge, but Luffy's a lot stronger now…