OnePlus may have started work on Pie for the 3/3T
In 2018, OnePlus announced its plans to update its flagship 3 series directly to Pie, bypassing Oreo (8.1) in the process. The OEM was lauded for this decision, particularly as the phones in question were already 2 years old at the time. Since then, this schedule has been officially delayed once, and the series is still on security patches that are 3 months old at this stage. Therefore, it may seem like the Chinese company has forgotten about its 2016 models after all.
However, this may not be the case. A user on the OEM's Chinese community website has claimed that there is a build of Hydrogen OS (H2OS) for one of these phones (the 3T) based on Pie. H2OS is the Android skin developed for OnePlus phones used in China, whereas Oxygen OS (OOS) is its otherwise global counterpart. H2OS has been found to often be the first to see new updates and features.
Therefore, it is possible that Pie for the OnePlus 3 and 3T is currently in the works. The forum user has also posted what appears to be a screenshot of the About page for the new 3T H2OS build, but also maintains that it is intended as a mock-up. Then again, speculation about H2OS has often translated to the appropriate OOS counterparts with at least some accuracy. If this one is correct, this build of H2OS should have passed beyond the testing stage by the end of February 2019.