As we pointed out the other day, the Google Pixel 3a and Pixel 3a XL both launched with an older version of the Android security patch pre-installed. Instead of the May 5th patch, which is found on all of the high-end Pixel models, the new mid-rangers come with the March 5th patch instead. And if you look at Google's website for the Android Beta Program (via 9to5 Google) you can see that the new models are not included in a list of Pixel phones eligible to receive the Android Q beta previews. The good news is that both issues will soon be resolved.
The same page on the Android Beta Program website reveals that the Pixel 3a and Pixel 3a XL will both be able to install Android Q beta releases starting next month. The Android Q fourth beta is expected early in June; that leads us to believe that the new mid-rangers will receive the June Android security patch at the beginning of next month at the same time that it is sent to the rest of the Pixel family.
Even though the Pixel 3a and Pixel 3a XL aren't officially able to install Android Q beta previews, it appears that some Pixel 3a users have been able to install the latest beta preview from the aforementioned Android Beta Preview website. To see if this is possible, use this link and click on "View your eligible devices." If your Pixel 3a or Pixel 3a XL shows up, you are in business. If not, you have about three weeks to wait.
Keep in mind that beta versions of Android Q, like any software, are not stable. They can be buggy and some of the features that you are used to with Android 9 Pie might not work. You might just want to wait for the final version of Android Q, which is due out in the third quarter of this year.