Volume button not working

You'll have to figure out whether the problem is with the physical button on the side of the phone that you press, or the tiny electrical switch mounted on a flex cable inside the phone.

If it's the button, the only way I'm aware of to repair or replace it would be to replace the midframe, as they all come with the buttons already installed. Unfortunately, there don't seem to be any midframe guides available, but the preferred way to replace the screen is to replace the screen+midframe as a single assembly, and there are guides for that. This guide will show you how to do that kind of screen replacement; once you've finished the guide you'll have just the screen+midframe assembly left.

Manuales / Poco X3 NFC / Fullscreen - Nadie Me Llama Gallina

Once you get that far, you can use the iFixit guide for the X3 Pro to remove the screen from the midframe, allowing you to replace the midframe by itself. Just start on Step 17 where you peel up the display connector, then follow the rest of the guide to take off the screen.

Poco X3 Pro Screen Replacement - iFixit Repair Guide

It appears that the X3, the X3 NFC and the X3 Pro all use the same midframe, so these guides should give you all the information you need.

If, on the other hand, the problem is with the internal flex cable and the electrical switch mounted on it, you'll need a different guide showing you how to replace just that part. This guide should be just what you need.

Manuales / Poco X3 NFC / Side push buttons - Nadie Me Llama Gallina