Marco only fought the two of them for a brief period of time— It was when he sent Zoro to the top and when Queen tried to attack Hyogoro to be exact. Other than that, He was mostly locked in a one-on-one with King. Queen stopped fighting Marco and focused on Chopper even before Pero entered the fight. Marco also stopped fighting the two of them. Correcting me if I am wrong but we never even saw Pero attacking Marco.
King is holding his own against Marco's best forms of fighting without using his own. I agree Marco lost a major chunk of his flames which results in less HP for him, but he is clearly inferior to King as a fighter because he couldn't fully dominate King despite him fighting Marco in the base for the most part. King took the best moves Marco has shown with little to no effort. His attacks are affecting Marco despite his regeneration. All in all, King clearly established that he's fully capable of besting Marco and his broken abilities. I am sure Marco can do much more if it's a proper one-on-one but he'll lose eventually.
King is holding his own against Marco's best forms of fighting without using his own. I agree Marco lost a major chunk of his flames which results in less HP for him, but he is clearly inferior to King as a fighter because he couldn't fully dominate King despite him fighting Marco in the base for the most part. King took the best moves Marco has shown with little to no effort. His attacks are affecting Marco despite his regeneration. All in all, King clearly established that he's fully capable of besting Marco and his broken abilities. I am sure Marco can do much more if it's a proper one-on-one but he'll lose eventually.
As per Perspero, Marco was already looking pretty exhausted, and that is backed by Marco's statement that holding off two people over a billion berries is pretty tough.
So even going into his supposed "1 v 1" against King, Marco was already heavily exhausted because of fighting against both King and Queen, meanwhile, King was still pretty fresh due to having Queen by his side helping wear down Marco.
And then we can clearly see Perospero joined in the fray, and was attacking Marco from a distance, even though Queen left the fight. So it was still technically a 2 v1, but just switched to long range combat for the other player.