https://www.techradar.com/news/google-pixel-3-pre-orders-apparently-start-on-october-9

We already know that Google is going to announce the Google Pixel 3 and Google Pixel 3 XL on October 9, but it looks like you’ll also be able to pre-order the handsets immediately after the event.

At least, that’s what Android Central says, having claimed to get a promotional email from Google with that information in it. We have no reason to doubt the site, so this is probably accurate, but we haven’t seen the email ourselves so can’t be completely certain that it’s true.