The 3 and 3T were the OnePlus flagships two years ago. Despite this, their manufacturer appears to have committed to update them to the latest version of Android. The phones currently run stock Oxygen OS (OOS) based on Oreo 8.0.
https://www.notebookcheck.net/OnePlus-3-and-3T-will-now-not-get-8-1-but-they-will-get-9-0.320026.0.html