After the Google I/O keynote, there was some confusion about how many devices would be getting Android Q Beta 3. A total of 21 was given originally, but that didn't include the newly announced Pixel 3a and 3a XL even though we'd separately been told they were going to get it. That was later confirmed, but they've now been removed.
As pointed out by some eagle-eyed Redditors, the official page on which you can enroll for the beta only lists support for the 6 previous Pixel phones right now, but a note has been added that reads "Pixel 3a and Pixel 3a XL will be eligible in June." Neither phone is listed on the Android Developers Site either. We're not sure why their access has been revoked, but at least we only have to wait until next month before they should return.
Some users were able to opt into the beta on these devices early and are now reporting issues trying to downgrade to a stable Android Pie build after unenrolling, with no automatic update forthcoming and manual installing proving problematic — so that's something Google needs to look into. Curiously, the Q Beta 3 system images for the 3a and 3a XL are still available to download and install (at the time of writing), so someone clearly didn't get the memo.
Pixel 3a and 3a XL Q Beta images now gone too
If you were planning to sideload the Q Beta 3 OTA on your Pixel 3a or 3a XL, this workaround has also been closed off by Google. The images are no longer available for download on the Android Developers site, so you'll now have to wait until next month to try the beta on those phones.
Correction: As pointed out in the comments, you can still manually flash a Q beta system image (available here) to a Pixel 3a or 3a XL. Beware, though, you will need to be enrolled in the beta to receive automatic OTA updates.
- Android Developers