Well, I can't say for sure, but generally when something like this happens we look for what the pieces have in common. In this case all three of the buttons you're having a problem with are on the same flex cable so the obvious place to start would be to replace the volume/power button flex cable.
Unfortunately, iFixit doesn't have a guide showing you how to do that repair, but I did locate one on a Spanish site called Nadie Me Llama Gallina.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/google-580/google-pixel-3a-1124/pulsadores-laterales-9913|Manuales / Google Pixel 3a / Side push buttons|new_window=true]
The part itself is pretty easy to get; I usually start out checking sites like Amazon, eBay and AliExpress. Here's the first one that come up on my cursory search.
[link|https://www.amazon.com/Power-Button-Cable-Google-Pixel/dp/B08MW993CT|Amazon.com: Power Volume Side Button Flex Cable for Google Pixel 3A G020A G020E G020B 5.6" : Cell Phones & Accessories|new_window=true]