They are supposed to release may, April and June's security patches sometime this month.
https://9to5google.com/2019/05/10/pixel-3a-security-patch-old/
Thank you
Here's the quote from the article:
"Update 5/10: Google has confirmed to 9to5Google that the Pixel 3a will be getting an update to combine the April, May, and June security patches together on June 3rd. That’s quite a ways away, but Google says this is to “ensure the best customer experience.” Further, the company said that the pre-loaded software is based on the manufacturing schedule."