KH1: While Sora is fighting for his life against dangerous monsters who want to NOM his soul and Riku gets ANSEM'd, Kairi is unconscious for most of the game, and stays in a safe area for the rest.
CoM: Sora fighting for his life to retain his memories, Riku is fighting to maintain control of his inner darkness, and Kairi is back home going to school.
358/2: Roxas and Xion sweep through hordes of heartless all year. Kairi still going to school.
KH2: Sora recovering from his coma and fighting for his life against monsters AND emotionless warriors who require more skill and strategy to defeat than 'whack it good'. Kairi kidnapped, is rescued and escapes without having to fight any of the really dangerous things lurking about.
DDD: Sora and Riku are both good enough to try for the Mark of Mastery. Kairi starts training, most likely in a controlled environment and not in pure life or death situations like Sora
KH3: Sora and Riku grind themselves to the absolute peak of their abilities to fight 13 of the most dangerous foes the universe has to offer being led by one of the most powerful and dangerous Keyblade Masters in recent memory. Kairi has at MOST actually fought some of the more common heartless, MAYBE a few of the stronger ones, putting her somewhere near mid-game KH1 Sora at best.

"WHY IS KAIRI SO WEAK AND USELESS?!?!?"

I can get why she wasn't able to do much in 3, but that doesn't make me any less annoyed that she keeps on getting damsel'd in the series and constantly has to be off screen barely doing much. Like, would it have killed them to show her training with Lea?