There have been several reports on Google’s product forums and Reddit where owners of the Pixel 3 are reporting that when they launch the camera, it just shows them a blank screen. Only a reboot of the phone returns the camera back to normal, although we imagine how inconvenient this must be.
It also seems that the trigger that causes this bug is when users attempt to launch the camera via apps, such as through Android Messages, Facebook, and barcode scanner apps. As far as a fix is concerned, a user on Reddit revealed that a Google support staff told them to perform a factory reset which doesn’t really seem to solve the problem either.
Google has yet to officially acknowledge the issue so it is unclear if a fix could already be in the works. In the meantime for Pixel 3 owners, if you want to use the camera, launch it via the app and not through other apps for now. Some users have had luck sending their phones in for a replacement unit, so you could try that too if your device is affected by the same bug.