I just seen Sony is rolling android N preview for the unlocked z3 and us tmobile z3 owners are not even on marshmallow yet. I think us tmobile z3 owners was forgotten about, I think the writing is on the wall that this varient of the z3 is dead.
As I understand it, Sony is getting out of the USA cell phone market. I would not expect any further carrier branded releases from them. In any case, only the Software updates page will tell you, and as of now, no update is listed.