In the Frozen trailer, we see Aqua revealed to have silver hair and yellow eyes. It appears that she has become a Xehanort, but if she is (I still think that there is a possibility that she isn't), then that means that Terra is no longer a Xehanort.
At the end of Dream Drop Distance, we see that all of the Xehanorts have already been chosen except for one. Sora was supposed to be the thirteenth Xehanort, but he was rescued at the last moment, which means that, at the beginning of Kingdom Hearts III, Xehanort still needs one more vessel. I don't think that Xehanort found his thirteenth vessel between Dream Drop Distance and Kingdom Hearts III due to the dialogue that he been revealed in the trailers so far, especially in the E3 Showcase trailer when Xemnas tells Sora, "Nothing would please us more than Roxas' return, of course." However, I do not believe that Aqua is the thirteenth vessel.
In the E3 showcase trailer, we see Riku and King Mickey return, which means that they must have finished their mission in the Realm of Darkness. In this scene, Sora is still wearing his Kingdom Hearts II outfit, which means that this scene must take place before he gets his new Kingdom Hearts III outfit. However, when Xemnas tells Sora that nothing would please them more than Roxas' return, he is wearing his new Kingdom Hearts III outfit, which must mean that, if Aqua is a Xehanort, she already became a Xehanort by the end of Dream Drop Distance and was sitting on a throne in Where Nothing Gathers with the other Xehanorts. If this is true, then Terra can no longer be a Xehanort.
After King Mickey lists off all of the Keyblade wielders he knows (himself, Sora, Riku, Terra, Aqua, and Ventus), Master Xehanort says to him, "But Sora and another on your list belong to me now." Master Xehanort claims that, in addition to Sora, one of the other Keyblade wielders on King Mickey's list is a Xehanort. This person obviously isn't Riku or King Mickey, so they have to be one of the Birth by Sleep trio. I always assumed that he was talking about Terra, but what if he wasn't? What if he was actually talking about Aqua. After all, even though he proudly announced earlier in this scene that Terra became his new vessel back in Birth by Sleep ("...and the feckless youth who became my new vessel"), he didn't say, "Sora and Terra belong to me now." He was a lot more vague with who the other person was. Why would he be vague if the other person was Terra?
So, if only one person from the Birth by Sleep trio is a Xehanort, and if that person is Aqua, then Terra can no longer be a Xehanort. However, in addition to Master Xehanort not revealing who the other Keyblade wielder, in addition to Sora, from King Mickey's list is, there is something else in this scene that seems to hint at the possibility that Terra is no longer a Xehanort. When Ansem and Xemnas were destroyed, Terra-Xehanort should have returned. After all, it was Terra-Xehanort that willingly gave up his heart and became a Heartless and a Nobody. However, we do not see Terra-Xehanort return, but Master Xehanort. From what I can gather, there are two explanations for this. Either Terra-Xehanort only gave up Master Xehanort's heart and not Terra's heart, which meant that Terra's heart was set free, or Terra finally found a way to break free of Master Xehanort's control. Either way, the end result is the same: Terra is free of Master Xehanort.
However, if this true, then where has Terra been all this time? Maybe, like when Ansem, Seeker of Darkness, banished Riku's heart to the Realm of Darkness in Kingdom Hearts 1, Terra is trapped in the Realm of Darkness. Maybe, if Aqua is a Xehanort, Terra shows up to save Riku and King Mickey from her in the Realm of Darkness. Maybe what started off as a mission to save Aqua from the Realm of Darkness ended with Riku and King Mickey saving Terra from the Realm of Darkness. Maybe, at the end of the scene in the E3 Showcase trailer where Riku and King Mickey return, we see Terra enter the room with them instead of Aqua. I guess we will have to wait and see.
In the Frozen trailer, we see Aqua revealed to have silver hair and yellow eyes. It appears that she has become a Xehanort, but if she is (I still think that there is a possibility that she isn't), then that means that Terra is no longer a Xehanort.
At the end of Dream Drop Distance, we see that all of the Xehanorts have already been chosen except for one. Sora was supposed to be the thirteenth Xehanort, but he was rescued at the last moment, which means that, at the beginning of Kingdom Hearts III, Xehanort still needs one more vessel. I don't think that Xehanort found his thirteenth vessel between Dream Drop Distance and Kingdom Hearts III due to the dialogue that he been revealed in the trailers so far, especially in the E3 Showcase trailer when Xemnas tells Sora, "Nothing would please us more than Roxas' return, of course." However, I do not believe that Aqua is the thirteenth vessel.
In the E3 showcase trailer, we see Riku and King Mickey return, which means that they must have finished their mission in the Realm of Darkness. In this scene, Sora is still wearing his Kingdom Hearts II outfit, which means that this scene must take place before he gets his new Kingdom Hearts III outfit. However, when Xemnas tells Sora that nothing would please them more than Roxas' return, he is wearing his new Kingdom Hearts III outfit, which must mean that, if Aqua is a Xehanort, she already became a Xehanort by the end of Dream Drop Distance and was sitting on a throne in Where Nothing Gathers with the other Xehanorts. If this is true, then Terra can no longer be a Xehanort.
After King Mickey lists off all of the Keyblade wielders he knows (himself, Sora, Riku, Terra, Aqua, and Ventus), Master Xehanort says to him, "But Sora and another on your list belong to me now." Master Xehanort claims that, in addition to Sora, one of the other Keyblade wielders on King Mickey's list is a Xehanort. This person obviously isn't Riku or King Mickey, so they have to be one of the Birth by Sleep trio. I always assumed that he was talking about Terra, but what if he wasn't? What if he was actually talking about Aqua. After all, even though he proudly announced earlier in this scene that Terra became his new vessel back in Birth by Sleep ("...and the feckless youth who became my new vessel"), he didn't say, "Sora and Terra belong to me now." He was a lot more vague with who the other person was. Why would he be vague if the other person was Terra?
So, if only one person from the Birth by Sleep trio is a Xehanort, and if that person is Aqua, then Terra can no longer be a Xehanort. However, in addition to Master Xehanort not revealing who the other Keyblade wielder, in addition to Sora, from King Mickey's list is, there is something else in this scene that seems to hint at the possibility that Terra is no longer a Xehanort. When Ansem and Xemnas were destroyed, Terra-Xehanort should have returned. After all, it was Terra-Xehanort that willingly gave up his heart and became a Heartless and a Nobody. However, we do not see Terra-Xehanort return, but Master Xehanort. From what I can gather, there are two explanations for this. Either Terra-Xehanort only gave up Master Xehanort's heart and not Terra's heart, which meant that Terra's heart was set free, or Terra finally found a way to break free of Master Xehanort's control. Either way, the end result is the same: Terra is free of Master Xehanort.
However, if this true, then where has Terra been all this time? Maybe, like when Ansem, Seeker of Darkness, banished Riku's heart to the Realm of Darkness in Kingdom Hearts 1, Terra is trapped in the Realm of Darkness. Maybe, if Aqua is a Xehanort, Terra shows up to save Riku and King Mickey from her in the Realm of Darkness. Maybe what started off as a mission to save Aqua from the Realm of Darkness ended with Riku and King Mickey saving Terra from the Realm of Darkness. Maybe, at the end of the scene in the E3 Showcase trailer where Riku and King Mickey return, we see Terra enter the room with them instead of Aqua. I guess we will have to wait and see.