OnePlus 3 has already received a number of beta updates while 3T will be getting it's first update today. Also as per Carl Pei's tweet, not every OnePlus 3/3T user will be getting the Nougat update. It will be gradual and maybe country based update rolling out starting tonight.
2) Android N stable OTA for the OP3 and the OP3T. Gradual rollout will begin later tonight.— Carl Pei (@getpeid) December 31, 2016
Pei stated, “Many members of our R&D are not taking any time off to celebrate New Year. But we all feel proud to finally deliver on our promises. Thanks for all the support in 2016. I promise we’ll come back stronger in 2017, see you next year!”
If you are eager to get your hands on the Beta update without waiting for the stable release, head over to the forum to get it.