A Pixel 3 XL case leak shows off the phone's single rear camera and the notch on the front of the device, and although this isn't the first time that either of these traits of Google's upcoming phone have been rumored or hinted at through leaks, it certainly adds more evidence that Google will indeed be keeping the rear-facing camera on both the Pixel 3 and Pixel 3 XL to a single sensor. As for the notch, Google has already more or less confirmed that a notch would be on the Pixel 3 XL with its addition to the Android P software for notch support.
A couple things to note about the leaked image is that it's less about the case and more about the image shown of the front of the Pixel 3 XL. The case cutouts show one single camera on the back along with the hole for an LED flash, as well as the fingerprint sensor, and although it looks like there's a hole at the top where you might find the 3.5mm audio port, this is likely just an opening for something else entirely and not the audio port, as Google removed the audio port on the Pixel 2 and Pixel 2 XL, and there's nothing to suggest that Google will be bringing this back.
Worth keeping in mind is that while the representation of the Pixel 3 XL in the leaked image for the case is quite like earlier leaks of the device in terms of design, the design of the phone has yet to be confirmed and anything, even minor details, could always change leading up to the eventual announcement and release of the phone. Google is suspected to be announcing the device later in the Fall, though rumors have also suggested that it may end up unveiling the phone earlier than it announced the Pixel 2 and Pixel 2 XL last year.