If you float over to T-Mobile's support page for software updates, you'll find a list of the devices which are scheduled to get the latest version of Android software, in this case Android Lollipop. Most of the devices are listed as completed because T-Mobile has already finished pushing out the updated software to subscribers or are in the process of completing the roll out. There are a few devices though that are either still in T-Mobile testing, or at the manufacturer development stage which means T-Mobile and the OEM who makes the device have agreed to push out the update but don't have a specific time frame for when they will roll it out to users.
The Sony Xperia Z3 T-Mobile model is one of these handsets, still sitting under the manufacturer development tag. The T-Mobile model, or D6616 if you go by the model number of the device, is mostly the same as the international and unlocked US models save for some of the T-Mobile software and it's likely this minor difference that is resulting in a longer waiting period to get the Lollipop update onto the phone. Whatever the case, the T-Mobile Xperia Z3 is left behind at this point while likely all other models of the phone have already been updated. Luck may be about to change for T-Mobile customers who are using the Xperia Z3 though as it seems the D6616 model has just been certified for the Lollipop software.
Noted by a listing over at PTCRB, the T-Mobile version of Sony's 2014 flagship device has been certified for software version build 23.1.C.0.381 with a date of May 8th. While this doesn't explicitly state anything about Lollipop, the change in build number from 23.0 to 23.1 in the first few numbers of the build software could signify this is a variant build of the Lollipop software, unfortunately even if it there is still no details regarding an actual date of when T-Mobile Xperia Z3 owners can expect to see the software pulled down to their devices. It could be more than a month or two out or could merely be weeks away.