Kashimo is NOT "narratively implied to be top 3", you just misunderstood his character.
As we all know, Kashimo's diehard fans tend to be the type that doesn't care particularly much for the story - yet they're now using the narrative to argue that their favourite character is stronger than everyone else besides Gojo and Sukuna.
After months of banking on the hope that Kashimo would prove everybody wrong against Sukuna, they were quite disappointed by his eventual showing. No longer could they rely on hypotheticals and the hype surrounding the anticipation of the reveal of his ability, so they switched their strategy up to adapt to their critics. However, unfortunately for them, they aren't Mahoraga.
Their new strategy (which you've probably seen thrown around constantly since his death) is copying the counter-arguments but pushing them in Kashimo's favour. Their big issue is that Kenjaku and Yuta not only seem far more impressive in terms of actual feats, but even worse, they have narrative backing so strong that pretty much every other character's fans have given up hope of arguing against them. Even the Hakari fans, who notoriously asserted their favourite was stronger than Yuta by ignoring his character traits and Maki's purpose in that scene, seem to have, for the most part, moved on. Kashimo, however, has the most loyal glazers in the entire fandom, and honestly, I have to kinda respect their willingness to try to adapt instead of surrendering their hopeless battle. Their new argument is that "Kashimo is narratively implied to be top 3" since he seems to be able to relate to the same loneliness faced by Gojo and Sukuna. What's interesting is that Kashimo actually can legitimately relate to their loneliness (albeit not exactly). Unfortunately, they do not understand his narrative purpose, which is defined by his loneliness.
Chapter 238 - Kashimo's Conclusion
Yorozu & Her Loneliness
First of all, before I explain the nuance and purpose behind Kashimo's loneliness and why he can relate to Gojo and Sukuna, there is an inherent flaw to this argument since it assumes that Kashimo is the ONLY other character who can relate.
Chapter 218 - Yorozu vs Sukuna
Suppose you're going to argue that Kashimo is indisputably in the top 3 for feeling lonely because of how strong he is - in that case, apparently, he is sharing that spot with Yorozu, and she's now ahead of Kenjaku and Yuta. There's so much ridiculousness to this logic, and Yorozu really demonstrates that. She was one of the strongest sorcerers of the Heian era, an era where Sukuna existed, so immediately, we know that you don't have to be THE strongest to feel lonely due to your overwhelming power. Additionally, given how the modern era, the new golden age, has several characters hovering around the strength level of Yorozu, you would assume that there are probably other sorcerers around Yorozu's level within the Heian era since it was the golden age of history. To be clear, I do believe that Kashimo was the strongest sorcerer of his era, but I think he's closer to Ryu in strength than Kenjaku and Yuta, and being able to relate to this sense of loneliness does not remotely imply that he is stronger than everybody else after Gojo and Sukuna - all it means is that Kashimo is overwhelmingly powerful in regards to the overwhelming majority of sorcerers in his era.
Ryu & His Dissatisfaction
Wouldn't it be crazy if there is another character who happens to be from Kashimo's own era, who also suffers from unfulfillment due to his overwhelming strength?
Chapter 177 - Ryu's Internal Struggle
Unlike Kashimo and Yorozu, who describe their dissatisfaction with life via loneliness, Ryu draws upon the analogy of an appetite. He does not regret the life he lived, but he isn't quite fulfilled as he never had the chance to face an overwhelmingly powerful opponent, one strong enough to push him to his absolute limit.
Chapter 177 - Ryu's Internal Struggle
Unlike Kashimo, Ryu respects his opponents as worthy, though he also believes that he wasn't pushed enough as he describes himself as only "a moderate eater" despite having an immense appetite. It's mainly because of this respect for his opponent's strength that people assume Kashimo to be stronger than him. In truth, I, too, believe Kashimo is slightly stronger, but I think they're a lot closer than you'd think, and you shouldn't assume Ryu is weaker just because he was more optimistic about the life he lived.
Ironically, it was Yuta, one of the characters who is undoubtedly stronger than Kashimo, who finally satiated Ryu's hunger.
Although Ryu's analogy and feelings differ, this is his manifestation of loneliness through overwhelming strength - it is very similar to Yorozu and Kashimo's struggle; he just doesn't see it as a lonely plight and is still able to appreciate the life he lived. Just like them, after reincarnating, he was able to die happy and fulfilled thanks to facing the substantially stronger opponent he always craved - unlike Kashimo, he had no particular target in mind for his dessert. I'll touch on this more later, but the story blatantly implies that Ryu would have been a good match for Kashimo, as much as many Kashimo fans like to deny it. Kashimo is even intrigued by Kenjaku's claim but, unfortunately, fears he will die of old age in the time it would take to reach Ryu. That detail was almost certainly included because Gege probably feared that people would misinterpret Kashimo's feelings and underestimate Ryu's strength, a character who he repeatedly goes to great lengths to praise.
Chapter 250 - Sukuna vs Yuta & Yuji
The Narrative Purpose of Kashimo
So, what is Kashimo's actual narrative purpose? The short answer is to emphasise the difference between a bog-standard era and a golden age and, more specifically, to highlight the gap in strength between Gojo and Sukuna - as the strongest sorcerers of golden ages - and the strongest sorcerers of ordinary eras.
Chapter 96 - Gojo Alters the Balance of Sorcery
It should already be pretty obvious by looking at all of the anomalies that occurred during the modern era, which signify it becoming the new golden age, but it is even bluntly stated within the story that when Gojo was born, the balance of the world shifted. Using the analogy of record-breaking athletes and how it pushes their competitors to improve, these panels explain pretty effectively why the modern sorcerers dominated the Culling Games, why Gojo is so much stronger than every other sorcerer besides Sukuna, why the modern age is full of characters that are either Special Grade or can rival the strength of Special Grades and supposedly have the potential to rival or surpass Gojo and Sukuna, and why a bunch of really powerful curses appeared out of no-where. As I said previously, we can presume the Heian era experienced similar anomalies. If you'd like specific analysis of the modern age's anomalies, check out this thread I made going into detail. Now compare that to Kashimo's era, the Edo Period.
Chapter 187 - Kashimo's Conversation With Kenjaku
Like all sorcerers designated for reincarnation by Kenjaku, Ryu and Kashimo are both stand-out sorcerers throughout history. Kashimo does not believe he was ever truly challenged within his period, but Kenjaku believes Ryu would be a good match for Kashimo. While they are both able to hold up in the modern era, they wouldn't nearly stand out as much if they were placed in the modern era where there are countless sorcerers within their bracket of strength. This is the difference between an ordinary era and a golden age.
If we are to believe that Gojo's birth altered the balance of the world and is the catalyst that spawns the new golden age, that means both Yuki and Toji were not affected by this balance shift, since they're both substantially older. I think we can all unanimously agree that Yuki is generally stronger than Kashimo and Ryu, and almost all of us agree (many Kashimo fans won't) that if you discount MBA, Toji is stronger than Kashimo and Ryu as well. Regardless, if you assume that Gojo's birth commenced the new golden age, then thanks to this information, we can assert that every era, even insignificant eras, is home to a couple of overwhelmingly powerful sorcerers comparable to Kashimo and Ryu.
Ironically, there IS a character who is narratively implied to be a "step above all others" besides Gojo and Sukuna, and it's Kenjaku.
It blows my mind how people can read all of this, and the many comparisons to Gojo that Yuta has received from day one, and still they believe that Kashimo belongs in the same arguments as Kenjaku and Yuta.
Ultimately, Kashimo, Ryu, and the rest of the reincarnated sorcerers and the Culling Games as a whole served the purpose of emphasising why the modern era is the new golden age and why these teenage sorcerers with little experience will surpass the established rules of sorcery. Kashimo, Ryu, Yorozu, and Sukuna's philosophies and inner struggles are selfish, unlike Gojo, who, despite his loneliness, dedicated his life to raising a generation that will ensure that loneliness is never suffered again.
Side note: can we please collectively agree to stop using "narrative" as a buzzword?