OnePlus 3T is a newer version of OnePlus 3 smartphone.  It has LCD screen with Super AMOLED panel which is not present in OnePlus 3.  It has a zippier snapdragon 821 chipset, 16 MP rear camera with Sony IMX398 sensor and a wider f/1.7 aperture better than f/2.0 aperture which is on the OnePlus 3.  The battery is increased from 3000 mAh to 3300 mAh.

There is a teaser from OnePlus weibo page showing a drawing on the back of the OnePlus 3T when the device is taken infront of the mirror. It looks exactly same as the OnePlus 3 device with just a T badge on it.
The teaser says “To Be a Better Me #NeverSettle & NeverStop.  OnePlus 3T is expected to be available on November 15^th 2016.