This is one of the reasons the OnePlus lineup is a cult hit. OnePlus devices are incredibly developer friendly. They were built on the fact that they supported CyanogenMod out of the box. In fact the OnePlus One was one of the first devices to run CyanogeOS. The all new OnePlus 3T is starting to land on doorsteps. The good new is that there is already a fully working version of TWRP for the device. With TWRP installed you can easily root the phone by simply flashing the SuperSU.zip. Eventually as developers build their roms you will be able to flash roms to the device. Be careful when you root this device though. The OnePlus 3T has dm-verity so if you swipe to "allow system modifications" without the intentions to root with SuperSU you won't be able to boot back into the phone without flashing a stock image. You can grab TWRP for the OnePlus 3T from the link below.
via XDA