Back in 2016, Android software updates were not that common. Samsung, HTC, LG, Sony, and Motorola, basically almost all brands, were struggling to update their phones on time. OnePlus was a small startup in 2016 coming off the heels of a failed OnePlus 2, and had to get OnePlus 3 on the ladder of success. One thing that OnePlus promised in 2016, that most of the brands were not even willing to talk about, was guaranteed timely updates.
OnePlus promised on 2 major Android platform updates, and 3 years of security updates to OnePlus 3 and OnePlus 3T (which was launched 6 months after the launch of OnePlus 3). As per the OnePlus’ software maintenance schedule, October 2019 security update for the OnePlus 3 and 3T will be the last scheduled updates for these models.
These phones are currently running on Android Pie, and will not receive any further Android version updates. OnePlus has done a good job in delivering 3 years of security updates to the phones. The final update will be rolled out in phases, so you should be on the lookout for the refresh.