Why does part of my touchscreen not respond to touch?

Unfortunately screens are especially susceptible to liquid damage and it sounds like the digitizer part of your screen has suffered permanent damage as a result. On modern phones the touch screen and the display are fused together and are not generally considered repairable individually; you have to replace the entire screen if either fails.

On your phone there are two ways to replace the screen; either with or without the frame. If you choose to replace just the screen without the frame, then here's the iFixit guide to that repair. Note that it's destructive; removing the old screen will destroy it completely and you will have no choice but to replace it after that.

Google Pixel 3 Display Assembly Replacement - iFixit Repair Guide

The other way to do it is the more official/recommended method, which is to replace the screen plus frame as a single assembly. That way you don't have to mess around with glue to resecure the screen onto the frame, but it does mean you have to basically gut the phone and transfer all of the interior components to the new frame. Honestly, both methods have their advantages and disadvantages, but with the screen+frame method you'll never have to worry about the screen falling off because you didn't get it glued back on correctly. Here's the guide for that repair.

Google Pixel 3 Screen Assembly Replacement - iFixit Repair Guide

The replacement part that iFixit sells has the frame attached, so you'd use the second guide if you want to support iFixit by purchasing your screen from them. The Fix Kit version is a good choice if you don't already have the tools you need, as it has all the tools and parts required to do the repair, including the precut adhesive sheet for gluing the rear panel back on the phone when you're done. Of course, you can always go online and find replacements of both kinds available on sites like Amazon, eBay and AliExpress, among others.

Hope that tells you what you need to know; feel free to ask questions and be sure to let us know how it all turns out!