Looks like your phone has fallen into a DIRBS loophole, and by the look of things, there isn't much you can do about it.
The Pixel 3 has 2 SIMs, technically speaking. But in reality, they can't be used simultaneously as the Pixel doesn't support DSDS (dual SIM, dual stand-by), so it's basically a single-SIM phone.
Now sure, ESIMs have an EID number (Embedded SIM Identification), instead of an IMEI. But DIRBS doesn't seem to support EID numbers yet, by the look of things.
All you can do is give the same IMEI twice in the DIRBS portal and pray it work.