It wasn’t that though, where Oda felt the need to distinguish their individual power, it was just Nami (iirc) calling them monsters. Everything after that really just became stuff the fans used to continually stick them in a box. In fact, to your last point about the indicator that they were closer together than they were to the rest of the crew, the douriki actually refuted that because the douriki of the enemies Zoro and Sanji faced were actually closer to the enemies the rest of the crew faced than they were to Lucci.
Agreed with the end sentence there, with the caveat that the “power thing” was more of a reference to Nami thinking they’re just freaks. It wasn’t the be-all-end-all, but fans took it and tried to make it so.
Completely agreed with this, though honestly you could point to earlier if you wished, EL for example. We always knew that every enemy Luffy faced prior to EL was stronger than the next strongest enemy by a fair margin, but EL was the first time we were definitively able to see just how much because of the douriki. And then it was immediately highlighted in the next arc with Luffy vs Moria, as well as some commentary from the crew themselves in that arc, as well as when they got separated.
Jinbe further breaks it, as you mentioned, though I don’t think assuming he’s weaker than either of them (Zoro is probably debatable, though) is even feasible. But even then assumption doesn’t stand, like you pointed out.
Overall, it’s something that doesn’t mean what the people who champion it want it to mean. It’s largely irrelevant, and always has been.
The fact that Oda felt the need to call them out like that and distinguish the three over the rest of the crew is enough.
The Douriki did not do that actually because 1) Zoro and Sanji high end of mid diffed their opponent, while for Luffy it was actually the most extreme fight he had so far, with both going blow to blow and leaving everything on the table. You can guess what kind of opponent Luffy would same diff as Zoro and Sanji did Kaku and Jabra respectively, but it´s definitely closer than Fukurou and Kumadori were to the latter two (Franky same diffed Fukurou like Zoro and Sanji did their opponents, Chopper extreme diffed Kumadori) and equally important 2) the three strongest of CP9 all had Zoan fruits that significantly increased their physical abilities, which was pointed out several times, compared to Kumadori and Fukurou, who did not have any DFs, and Kalifa and Blueno who had more assistance type DFs. To add, both Lucci and Jabra had predatory type Zoans, which even additionally adds to it.
So yes, the three being closer than the crew are to them has always been true pre-TS, obviously before Franky and Robin even joined, but also after that. I would say around high diff opponents for Luffy.
Post-TS, that obviously changed. Sanji was barely matching opponents who were in the past and still are getting toyed with by the level of Luffy´s opponents, while Zoro was somewhere in-between these two (i would say Luffy was mid high tier, Zoro low high tier or high mid tier, Sanji mid mid tier, and Franky low mid tier). So obviously monster trio did not make much sense anymore.
And then Luffy even went further ahead with FS, high high tier, while everyone else remained the same. And with Jinbe joining, who himself is a low high tier, maybe even more, the concept became utterly meaningless.
With Sanji getting the RS, he jumped significantly up, but it´s debatable if he can advance from Jinbe more than the gap between him and Luffy, which is still really big, especially with Luffy learning another advanced Haki technique and moving up to top tier levels.
So right now, Jinbe, Sanji and Zoro are all pretty close to each other, while Luffy is significantly ahead, and the gap between Sanji/Jinbe and Franky is also very big, hence why it makes more sense to group these three together now.
What is gonna happen in the future can be debated, but i still do not see how Sanji could remain closer to PK level Luffy than to high tier Jinbe.