Google just posted the first factory image and OTA files for the Pixel 3 and Pixel 3 XL. Since today is basically launch day, the timing seems just about right.
The files are available as Android 9.0.0 builds PD1A.180720.030 and PD1A.180720.031 for both phones, the second being the Verizon specific build. This is the build you should find out of the box.
These contain the September security patch, for those curious, which is odd knowing that we’re almost to November. Google could either push an update before November to get us on October’s or they may wait and just give us November’s. We’ll have to see how that goes.