Kirby>Sonic>Mario.
Mario: He does not have many abilities for his own compared to the other two. He obligatorily requires his items to have a minimal variety of movements and when he run out of them he will be dead, his physical strength will be useless in a battle determined by haxing.
Most of his abilitys are based on a element, weapon or just let him fly ( which his opponents have and more ) and his his best items like the golden flower, star, metal cap, giant mushroom and boo mushroom have time limit. I just simply can't see Mario winning.
Sonic: Unlikely Mario, Sonic have many ability in his moveset, has many transformations which can boost his stats and the same powerups as Mario and even more like gravity, electricity, explotion, absorption manipulation and teleportation, the problem is that most of them have time limit which is a pity.
But due to his transformations and own movements that Mario lacks, put him in par with Kirby.
Kirby: Finally we have the puffball, while he are on par with Sonic in the department on hax, he just ate the last cake here.
Has various elemental manipulation, animal manipulation, has weapon mastery and more, can copy the strengh of his foes, can combine his own abilities, upgrade them via robobot armor and final weapons, can summon partner, can inhale everything with miracle fruit, has immortality, resistance to reality warping, possession, magic and spatial manipulation, (pufffm, i'm tired) and does not need to worry about a time limit -w-.
I'm Leaning to Kirby.
