OnePlus 3T gets TWRP custom recovery soon after market launch
If you've already owned a OnePlus smartphone before, then you already know that you will be able to keep your warranty even if you unlock the bootloader of the 3T and install third-party custom ROMs on the device.
You'll need to obtain root in order to flash custom ROMs, so make sure to install adb and fastboot on your computer, and then have TWRP downloaded, too. You'll want to follow all the instruction provided by folks over at XDA Developers forums who've put this together.
Keep in mind that this TWRP is not available for the OnePlus 3, so just use it on the OnePlus 3T if you already own one. It's also worth mentioning that the OnePlus 3T stock boot image has dm-verity, which will prevent users from booting if they swipe to enable system modifications.
In order to avoid that, simply follow the full rooting steps found by accessing the link at the source. That way you will prevent your phone from starting a boot loop.
TWRP 3.0.2-0 is the initial release for the OnePlus 3T and features MTP support, exFAT file system support along with NTFS, ADB root, built in Android 6.0 tree, as well as the option to flash OnePlus full OTAs.
There are some known issues as well, such as the fact that the OTG storage doesn't work yet, and the partitions encrypted with a non-default password won't decrypt.