Only reason I'm asking is cause he technically had 3 hearts inside of him. Terra, Master Eraqus, and Master Xehanort. So what happens when a person with multiple hearts turns into a heartless? I mean in Sora's case Roxas was made with Ven's heart, and as far as I know he is the only person to turn into a heartless with multiple hearts in him. So when Apprentice Xehanort turned into a heartless where did those hearts go?

As far as we know so far. One of the hearts was made to create a heartless- Ansem, seeker of darkness. This is presumed to be MX's heart, due to the similarities in their appearance and personalities. This has not been confirmed however.

Xemnas, is presumed to have a heart inside of him, as Noruma has confirmed that both Roxas and Xemnas are special (they can feel emotions) for the same reason. Since Roxas is special due to Ven's heart, Xemnas is thought to also have at least one heart inside of him. Probably Terra's, due to how Xemnas refers to Aqua's armour and keyblade as his friend. However, this has not been confirmed.

There is also some speculation that a second Nobody was made as well. The reason for it being that, Sora had three hearts in his body. Kairi's his own and Ven's. Kairi somehow got a Nobody when her host, Sora lost his heart, despite her body being in tact. But since we have little knowledge on how Namine came to be, this is also not confirmed and the most speculated of the three and by that I mean it has the least truth in it.

That's a very good question you've asked there. Not much has been mentioned about what happened to Terra, Eraqus, and Xehanort's hearts. But it is assumed that Xehanort's (and possibly Terra's heart) heart had turned into a Heartless that formed into Ansem SoD due to most likely the fact that Xehanort's heart had fused with Terra's body. But like I saod, it's only assumed that way. Nobody is truly sure of what happened to Terra, Eraqus, and Xehanort's hearts. Nor has it even been explained as to how and why Apprentice Xehanort's Heartless looks different from the  regular Pureblood and Emblem Heartless. This may be answered by BBSv2 at this point. As for the Nobody Xemnas, it is pressumed that he has a heart inside him,possibly Terra's heart since he is more related to Terra.

There's is three possibilities, 1: All three of them are part of Ansem SoD (with Xehanort being dominate) 2:: they became their own separate Heartlesses (eh.. doubt it) or 3: some other confusing thing that Nomura came up with

As far as we know so far. One of the hearts was made to create a heartless- Ansem, seeker of darkness. This is presumed to be MX's heart, due to the similarities in their appearance and personalities. This has not been confirmed however.

Xemnas, is presumed to have a heart inside of him, as Noruma has confirmed that both Roxas and Xemnas are special (they can feel emotions) for the same reason. Since Roxas is special due to Ven's heart, Xemnas is thought to also have at least one heart inside of him. Probably Terra's, due to how Xemnas refers to Aqua's armour and keyblade as his friend. However, this has not been confirmed.

There is also some speculation that a second Nobody was made as well. The reason for it being that, Sora had three hearts in his body. Kairi's his own and Ven's. Kairi somehow got a Nobody when her host, Sora lost his heart, despite her body being in tact. But since we have little knowledge on how Namine came to be, this is also not confirmed and the most speculated of the three and by that I mean it has the least truth in it.

 

Hmm, hadn't heard about that last bit. Maybe.

Anyway, I agree with Keyblade King 12 on this. Though I'm still wondering how Master Eraqus's heart works in all this.