@chrisporro Unfortunately with so many phones and other devices out there, iFixit has to prioritize which guides to create so it's inevitable that there are going to be some that are needed but aren't there. I ran into that same situation with the iPhone X and the Nintendo Switch and ended up authoring a couple dozen guides for just those two devices.
Anyway, I didn't have an luck finding a step by step iFixit-style guide for fixing your power button. However, I did find a YouTube video, but it starts with the assumption that most of the parts have already been removed. So here's my somewhat half-assed attempt at guiding you through this repair.
First off, I'm pretty sure you're going to have to remove the motherboard and the battery. I'd start with the battery just to be safe.
[guide|124702|Google Pixel 3 Battery Replacement - iFixit Repair Guide|new_window=true]
Then you'll be able to skip ahead on the motherboard guide and pull the motherboard.
[guide|124705|Google Pixel 3 Motherboard Replacement - iFixit Repair Guide|new_window=true]
Now that everything's out of the way, this YouTube video should get you through the process of removing and replacing the buttons.
[link|https://www.youtube.com/watch?v=GXoxEqMPyuQ|Google pixel 3 on/off volume button not working take apart and trouble shoot - YouTube|new_window=true]
Note that there are two parts to the side buttons; there's the electrical with the circuit board and the little dome clickers, and then there are the physical buttons that you push on. Either part could be at fault here and you may not be able to tell which it is until you get the phone opened up and the parts removed. It could be as simple as cleaning the buttons and their mounting holes, or it could be that the circuit board has failed and needs to be replaced. The parts shouldn't be hard to get; an internet search will turn them up easily enough. I usually start with Amazon, eBay and AliExpress, but pick whatever fits your delivery time vs. reputability vs. cost. Here are examples of each part; these are just the first ones that came up.
[link|https://www.ebay.com/itm/134122678226|Replace For Google Pixel 3 G013A G013B Side Volume Power Button Flex Cable|new_window=true]
[link|https://www.aliexpress.us/item/3256803159945710.html|For Google Pixel 3 3A 3XL XL ON OFF Power Volume Button Side Key Parts - AliExpress|new_window=true]
In theory that should get you going. Take lots of pictures as you work and use those to create your own iFixit guide for those who come after you with the same problem! It's fun and easy and you'll earn the appreciation of the community.