My new Pixel 3 (not the XL) is a super durable creature. In most day-to-day activities, I don't use a cover. After a month, it is still blemish-free.
However, it is super slippery. In fact, it is so slippery that it wants to slide out of the "X-Grip" phone holder on my motorcycle's handlebars. That means on trips where I want to use it as a GPS, I am live in constant fear it will fall out. On a motorcycle, that is certain death for the phone, because the next stop is pavement (or trail, I ride that way) and a tumble for dozens of yards.
The Otterbox Defender is perfect for this use. It is the opposite of slippery... it is grippy. It is also fat enough to make my skinny Pixel 3 wide for the X-Grip that I bought for my OG Pixel XL. In the several trips I've made so far, the Otterbox-encased phone hasn't budged a single iota.
I am not a huge fan of the belt-clip piece, because as a muffin-top, it bites into my overhang. It works perfectly, of course, and the fault is all mine by being big.
While I can fit the entire thing in my pocket, the aforementioned grippiness makes getting in and out of my pocket a nuisance. So, I have gained a lot of practice in taking the Otterbox off and putting it back on: off for day-to-day, on for "dangerous" situations like going to the zoo. There has not been any issue at all with fitment or excess tightness or sloppiness; the Otterbox hugs the phone just enough.
Speaking of the zoo, we went to see Christmas lights there. The Pixel 3 has this fabulous "Night Sight" feature that takes unbelievable photos in pitch black. See the example below. The point here is that the cover does not occlude any of the lens. The mode needs a second or more of the aperature being open, so if there was any part being blocked, you would see it cloudy. Thus, the image perfectly demonstrates that the cover openings do not impair photography at all. I've had covers that do, and I know the aggravation.
TLDR; the Otterboox Defender for the Pixel 3 is faultless.