Pixel 3 will only have gesture-based navigation, no traditional software buttons
Android 9 Pie is out today and it comes with Google's first ever gesture-based navigation system. That, however, is opt-in at the moment, for devices that are getting updated to Pie. You can enable it if you want, otherwise you're still able to use the traditional navigation with three software buttons - Back, Home, and Recents.
This situation will not remain the same going forward. For Google devices launching in the future, including the Pixel 3 and Pixel 3 XL which should be made official at an event on October 4, the traditional buttons will be gone.
Thus, the one and only option you'll get is the new gesture-based navigation with the center pill and Back button on the left (which appears only when it's necessary). The information has emerged through an interview by Android Central with EK Chung, Google's UX manager for Android handheld and Pixel.
The company chose to retire the multitasking button because user diagnostics showed that very few people actually used it on a regular basis. Throughout user testing performed on the new gesture system with "normal" consumers, Google found that their most-loved feature was the ability to quickly jump between apps by sliding the pill to the right.
As is par for the course in the Android world, just because Google has its own gesture system now it doesn't mean that the same exact one will be used by other OEMs. They are still free to ship handsets with the old three-button navigation, or even their own gestures like OnePlus and Motorola have already done. Fragmentation definitely isn't going away anytime soon.