According to threads posted on the Google Pixel Help forum and reddit, numerous people have encountered problems with the rear camera on the back of the Pixel 3 and Pixel 3 XL. The issue presents itself as the camera viewfinder wildly shaking when the user attempts to take a picture. This problem usually could be dismissed as "You're holding it wrong", with the user contributing to the shaking motions with unsteady hands. However, the exaggerated shaking within the viewfinder can even be seen when the camera is placed -- motionless -- on a table.
You can clearly see the odd behavior in the video below as the phone is shown seated on an upright stand:
Users that have complained about the problem report hearing noises coming from the rear camera module when all of this shaking takes place. Android Police, which was the first to compile all of the reports of the camera shaking, surmises that it could actually be a physical problem with the optical image stabilization hardware, or perhaps even the autofocus mechanism.
In addition, it appears that that the Pixel 3 is the most susceptible to the unwanted camera shaking issue. There are scattered reports of the problem affecting the Pixel 3 XL as well, but it is not nearly as widespread on these larger devices.
At this time, Google has not posted an official comment on this problem, so we don't know if a fix can be pushed out via a software/firmware update, or if this is a [failing] hardware problem that will need to be taken care off with warranty repairs.