Before even the time-skip took place I believed there was no M3 Luffy & Zoro delved into techniques which they didn't understand (Zoro CoA & CoO) (Luffy CoC & CoO).
Sanji on the other hand was lagging behind and didn't show any signs of haki (before Sanji fans do the ignore ᕙ(⇀‸↼‶)ᕗ) just read through this first.
Sanji is a strong fighter and he is capable of holding his own merit but I don't believe he is within the tier of Luffy's & Zoro's in fact I believe that the time-skip cemented this while Luffy and Zoro main focus is training their bodies and improving their haki , Sanji's focus was on supporting the crew through his cooking and also increasing his combat skills. (his main priority was his cooking)
For Zoro his Coa is his pride the second his sword is damaged its considered shame (Zoro and Mihawk discussed this)
For Luffy his CoC and his will to be the Pirate King is held in a similar regard.
Since Sanji has not shown any CoA (hardening) or any highly impressive CoO feats at the moment it seems like the cook is lagging behind especially since the Raid Suit is involved.
Advanced CoA FEATS :
Chapter 687 - We have Zoro cutting Monets cheek displaying that he is capable of sending haki air slashes
Chapter 955 - We have Luffy sending a Haki air punch towards a tree displaying that he's pretty much perfected Advanced haki
Luffy and Zoro have been using advanced CoA while Sanji hasn't shown any hardening.
What your thoughts on the current structure of the three Starwhats ?
People seems to forget that Oda has done a lot when it comes to the Luffy Zoro duo when compared to M3. Iirc there are more instances of the duo than there are of m3
Even how Luffy treats Zoro and Sanji is different.
When Luffy reunited with Sanji after the wci stuff he was just happy to see him
When Luffy reunited with Zoro after the wci, he was so happy he ran up to Zoro jumped on Zoro and hugged him. The only other guys he did this to were his brothers, ie Sabo.
The relationship between Zoro and Luffy is based on strength and friendship. Luffy made sure Zoro was strong enough before recruiting him. Zoro also makes sure Luffy isnt seen as weak. He did this withthe Usopp incident and in FI after Luffy showed CoC etc. Both of them dont allow ththe other to appear weak or to be weak.
Relationship between Luffy and everyone else is just based on friendship. Luffy didnt even bother to check on their strength. He just checked whether they were good people and whether he could be nakama with them
The gap between them will forever be small and they will forever be in the same tier and lvl