Earlier this year, T-Mobile removed the Sony Xperia Z3 device from their lineup, which meant it was pulled from in-store shelves and was no longer listed on their website. This was after having only been available for a mere six months on the network as a possible device for consumers, which is a rather short period of time for any phone especially one that is the flagship device of a manufacturer. Not more than a month later T-Mobile was once again listing the Sony Xperia Z3 on their web pages and was carrying the phone in stock as a device available for purchase, only then it was listed at a new lower price of $499 for the full retail cost down from the original $630.
As of now, it appears that the Sony Xperia Z3 has disappeared for the second time from T-mobile's online website as a listed product for sale, which could mean one of a couple different things. What's perhaps more odd though than the phone leaving the lineup again so soon after it was brought back, is that you can still order the original Sony Xperia Z or Sony Xperia Z1s. Of course they are listed as pre-owned devices and not new so that could have something to do with them sticking around while the most recent flagship is nowhere to be seen.
As for what this means, it's possible that T-Mobile has run out of stock and so they pulled the device. It's also possible that they are no longer going to be selling the phone. In light of the new Xperia Z3+ (Xperia Z4) being launched overseas they could be looking ahead, and may be preparing to pick up and sell the new device, although nothing has been confirmed for a U.S. launch except for Verizon Wireless, who has said they will be carrying the phone under the Xperia Z4v moniker. For now, if you're still looking at grabbing the Xperia Z3 and you use T-Mobile's network, it looks like your possibilities are limited to other online retailers like eBay or Amazon, both of which should have models still available. Although, it's worth noting these likely won't be the D6613 model which is what T-Mobile used to carry, and instead will be the unlocked D6603 or D6653 models.