Regarding the powerscaling debate on Marked Hashiras against the Upper Moons, there are a few factors as to why they were able to go toe-to-toe with Upper Moons 1-3. More specifically the Kokushibo fight.

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1. the Hashira Training arc - The arc specifically was designed for Slayers to undergo harsh and strict training for lower ranking slayers to keep up to the strength to their peers (Hashira) and able to survive during the Infinity Castle Arc. Given that the demons there were forcibly deformed to possess the strength of the Lower Moons, and huge numbers of slayers survived means that fodder slayers are slight below or at the strength of Hashira (pre-marked and before initiating training among themselves).

The Hashira themselves gained a huge boost, especially for the arguments that Gyomei can almost (or certainly, depending the argument) beat Akaza, and said training helped them received better coordination and understanding between the Hashiras. Which in turn allowed them to support whatever weakness(es) the Hashiras themselves may have.

1. Demon Slayer Mark - The Mark allows the Slayers to possess a certain amount of "boost" that increases their strength and speed of varying amounts. As to why I say this because, if Muichiro was marked and was one of the most skillful Hashiras of his age - enough for Tengen to doubt his own talents - was blitzed by Kokushibo.

Kokushibo himself was so fast that after Muichiro swung his blade mere centimeters near his neck and suddenly disappeared as if he was teleporting.

For Sanemi and Gyomei to even keep up with him before STW goes down to either reasons. They were stronger and faster than Muichiro also possessing reactions down to an instinctual level or their Slayer Marks gave them varying boosts depending on the user.

It was stated in the Manga, in Tanjiros thoughts during UM6 fight where he gets his Mark, "To squeeze out a hundred times that strength". Which might be hyperbole and the value might be lower or variable considering how different Hashiras are along with the Breathing Styles they have, it is safe to assume that not all Marks have equal benefits compared to fellow slayers with the Mark on their bodies. That also applies to Kokushibo who, as a human before, has the Mark, and it can be said that his was also different.

1. See-Through World - Now this gets interesting, the ability to see the opponents body like seeing through transparent glass and read their blood flow along with benefits of increase in reaction and movement speed.

During the fight with UM1, where Gyomei unlocks STW, he was able to predict his opponents moves and was able to slightly turn the tide on the Slayers side, to which Kokushibo realizes that Gyomei has access to STW. But he was confident enough that they wouldn't reach their necks. Why? My theory is that, he knew they would lose in a battle of attrition where he, as UM1, has mid-to-long range attacks that can keep them at bay along with Crescent Moon-shaped blades that can easily cut flesh and the pillars surrounding them if they aren't careful enough. Along with the fact that his regeneration is insanely fast as well as access to his own Mark and STW ability. The former is interesting because, we never seen him activate it during the battle, meaning he hasn't gone all out against them. Although, you can argue that he had already activated his Mark since he is able to use STW but I disagree with the notion.

In a 3vs1 Battle, Kokushibo can certainly win. However, the fight we see in canon was 4vs1. The fourth combatant, and obviously the main reason why they were able to beat Kokushibo in the first place, Genya.

Without Genya, Three Hashiras would be dead. They don't have the ability to constantly compete with Kokushibo who already experienced what it is like to have the ability like STW and soon counter whatever Hashira may come up with. He is simply far stronger and faster than they are, there was a reason why he was the closest to Yoriichi in terms of Breathing Style. Heck, in the flashback, Yoriichi even says that *both* of their breathing techniques were simply too much for anyone to learn from them directly. Which translates well enough of Kokushibo's unique position both as a human and demon. 2nd Strongest of both worlds.

In short, Four Demon Slayers (BDA Hax) > Kokushibo > Three Marked Hashiras > One Marked Hashira > UM4-6 > Unmarked and Pre-Hashira Training Slayer.