Sanji has alway been weaker than both Luffy and Zoro. This makes perfect sense because Luffy/Zoro are the fighters and that's the only thing they are good at. Not only is Sanji a cook, but he is a strong fighter himself. The latter two have been equally throughout the manga, except for when Luffy got gears. Sanji would give Zoro a hard fight, but he's not pushing him to the extreme. That's being virtually equals which they aren't. Zoro is always compared to Luffy, while Sanji never is.