The thing about the M3 is that they are just the 3 strongest SHs by a huge margin. The difference between Sanji and the 4th strongest SH at the time was huge. However, Jinbe complicates things. We've had cases of characters like Robin or Franky who seemed comparable to the M3 prior to joining the crew, but unlike them, Jinbe has a far firmer standing in the One Piece world, even before becoming a SH. Oda keeping him away might be an attempt to distance him from the M3. So if that's the case, the M3 stays.
I digress, moving back to the actual M3, Luffy and Zoro have always been more hyped and pulled ahead. Sanji literally joined the crew because he was inspired by their convictions. They were the first 2 to get bounties, marine counterparts in Smoker and Tashigi, be a part of the WG, trained by top tiers, parallels/connections with many strong characters like Roger, Rayleigh, Ryuma, Oden. Desires to surpass top tiers in Shanks and Mihawk. Even minor details, like Kaku and Lucci being the only ones still knocked out after their respective fights which were against Zoro and Luffy, respectively.
Sanji is strong, but his priorities and focus lie elsewhere. For starters, Sanji shines as a protector of the crew, looking out for the weaker members when things get rough, being able to influence the hearts of people through his kindness, getting more versatile roles thrust upon him, and never letting go of his virtues. These are the things that make Sanji special.