What don't you get, lol?
In Birth By Sleep, Xehanort stole Terra's body, so Terra basically disappeared. In KH 3D, when the one dude . . . I forget his name, apparently tries to restore Xehanort's memories, it's Terra that awakens, although he still looks like Xehanort.
Also at the end of Birth By Sleep, Ven's heart was sent into a "slumber" from which it may or may not awaken. Since Sora and Riku are going through the worlds in the darkness of their slumber in KH 3D, they might find Ven.
Again, at the end of Birth By Sleep, Aqua was apparently trapped in the Realm of Darness (you know that weird island with the black rocks in the water they occasionally show? That's it.) and I guess she doesn't age there, because a dude in an Organization cloak tells her about Sora and how he's saved the worlds multiple times now.
SO, they might bring those three back in KH 3D, or maybe a later installment. What I'm saying is: We can play around with that theory for the KH RP ;3