OnePlus 3 is very easy to root: official TWRP version released
While other phone makers are restricting access to their bootloader and making it hard to root your Android phone, the OnePlus 3 has proven to be an island of freedom: with an easily unlockable bootloader and now, an official version of TWRP for the OnePlus 3, rooting your phone is a piece of cake.
Of course, if you have never rooted your phone before, proceed with caution and if you're not sure what you need root for and would rather not deal with terminals, you probably should not root your device.
Let us recap the process of how to root the OnePlus 3. You first start, by unlocking developer options and allowing OEM unlocks and USB debugging. Then, you simply write down the following line "fastboot devices" and then "fastboot oem unlock" in the command line / terminal.
source: TWRP for OnePlus 3