OnePlus was among one of the first non-Google OEMs to roll out Android Pie for their devices. The OnePlus 5, 5T and 6 all received their update in late 2018. The OnePlus 3 and 3T, however, was promised Oreo 8.1 and today, the company announced a closed Android Pie beta for both devices. Shortly after announcing the HydrogenOS beta based on Android 9 Pie for the Chinese variants of the OnePlus 3 & 3T, OnePlus has confirmed a similar closed beta program for users in other regions.
In a post on the Official OnePlus Forums, OnePlus' community manager confirms that this closed beta test is open to anyone willing to signup until 2 pm GMT (10 am EST) on March 23. You can sign up for the beta using this link. These things usually have limited slots, and they tend to fill up quickly, so reserve your spot before its too late.
Before signing up, bear in mind that the beta isn't for everyone. As the name suggests, the software you'll receive will be clunky and full of bugs. A lot of apps may not work as intended and you'll very likely be required to flash your device multiple times. If you're ok with the conditions and are familiar with flashing builds, we'd still recommend that you don't use the device in question as your daily driver.
If you have signed up for the beta and are deemed eligible to participate in the beta, you should receive an email with instructions on how to proceed further. The program will likely last for a few weeks before ending. You may have to manually flash the final build upon release as OnePlus as historically segregated their stable and beta build paths. We can expect to see a final release roll out OTA in the next month or two, depending on how the beta goes.
News Source: OnePlus