[Update: Independent confirmation]
T-Mobile may start carrying the Pixel 3, 3 XL, 3a, and 3a XL
https://www.androidpolice.com/2019/04/15/t-mobile-may-start-carrying-the-pixel-3-and-3-xl/

[Update: Pixel 3a, too] Source:
Google Pixel 3 could soon be added to T-Mobile’s lineup
https://9to5google.com/2019/04/15/google-pixel-3-t-mobile/

Google teases new Pixel announcement for May 7th
https://www.theverge.com/2019/4/15/18312052/google-pixel-3a-announcement-teaser-io-2019-event-may-7