OnePlus co-founder Carl Pei confirmed (via Twitter) that a stable Android Nougat update is ready, and will be gradually rolled out, over the air, to the OnePlus 3T and OnePlus 3 "later tonight." Since we're talking about an over the air update, it's likely that not all OnePlus 3 and 3T owners will receive it today. But the update is coming, and everyone will probably get it in a few days the latest.
If you just can't wait for the stable release, and you know how to install a software update manually, a beta Nougat build is currently available to download via OnePlus' official forum. However, this particular beta is only for the OnePlus 3T - the OnePlus 3 is not included. For more details, check out the second source link below.
2) Android N stable OTA for the OP3 and the OP3T. Gradual rollout will begin later tonight.— Carl Pei (@getpeid) December 31, 2016
sources: Car Pei (Twitter), OnePlus forum