OnePlus has reached an important stage in the development of Oxygen OS 9, which is based on Android Pie (9.0), for its former 3/3T flagships. It now has a closed beta of this build, which can be downloaded following a successful application to the program in question. Therefore, the general, stable release of this ROM is closer than ever.
https://www.notebookcheck.net/OnePlus-shows-that-it-has-made-progress-on-Pie-for-the-3-and-3T.414916.0.html