I don't think there is a SP3 model that has LTE, I know of the Surface 3 (SP3's smaller brother). I haven't heard from T-Mobile for Personal accounts yet.
All the information I read said it they would start offering it to business customers on 7/31 but it doesn't look like that happened. I hope they do get it and offer it to consumers because I would love to have one, but just like they were supposed to get the Lumia 830, I will believe it when I see it...