Sora, Ventus, and Roxas?

Okay, I'll try and answer this as best as I can without rambling.

Ventus and Sora:

Ventus' heart bonded with Sora's. That's why Roxas looks like Ventus: when Sora was split into a nobody and a heartless, I think Ventus went with the nobody shell rather than the heartless shell. That hasn't been confirmed or denied, but it's my opinion.

Sora and Kairi:

Sora gave Kairi's heart BACK to her, that's why she woke up. When he stabbed himself with the Dark Keyblade, her heart that he was holding for her got released. So he became a heartless. I think he accidently gave her both his and HER heart. (When you stab yourself with a keyblade, I think it's tough to pick just one heart to give away.) I believe that when she hugged him, trying to protect him from the other heartless, she gave him his heart back. I believe the fragments of Ventus were released from him when he stabbed himself with the Dark keyblade.

I think all three of the hearts were relased. Kairi got two, gave one back to Sora, and the fragments of Ventus went into Roxas, Sora's nobody.

That is completely opinion based, and I hope it makes sense XD

ok if you have played kh 2 and kh358/2 days u find out that roxas is sora Other so i dont get where ventus came in but yet again i have not play khbbs and kh1 but sora became a heart less so kairi could have the darkness out of her heart