In My opion, I think that Kairi will be party member on KH3 fighting the Last Xehanort with Sora and Riku.
Review- Sora (along with Donald and Goofy0 fought against Ansem SoD, n Riku (who assisted Sora) fought against Xemnas. I mean think about it one of the three main characters fought and defeated the final boss to the end also on BBS, Ventus, Terra, n Aqua were the first 3 main chacters to be in party to fight against boss unversed depending on which of the three u play which also could hint that Kairi will be the 3rd to make her debut to fight in a major battle with her own keyblade.