@800XL I'm afraid, yes. You can try but there is no guarantees that it is possible at all. I have checked it across some forums and looks like it's pretty uncommon situation when Pixels are locked. Anyway, Pixel 3a is pretty cheap today (I've bought one in less than $100 with ideal condition) so if you have the version with encrypted bootloader, you can try to sell your current model and buy a new one without encrypted bootloader (probably it may cost a little more if you search specifically 🤷‍♂️)

@800XL I think guides even aren't needed) Before flashing Pixel 3a with UT, you will need to flash it to Android 9. The easiest way is doing it via Google's web-based flasher, you don't even need to download firmware to your PC. This flasher will download selected firmware automatically and will unlock bootloader also automatically.. or at least semi-automatically, you will still need to follow some steps that is displayed on your PC's screen (not phone' screen!).
Check UT site for firmware code

@dadarobot actually, Pixel 3a was announced at Google I/O at May 2019, so it's even less than 5 years old (if we count it right after announce) so you're lying right now.
P.S. do you have better ideas? Pixel 3a has 100% compatibility rate at UBports site and I personally use it as my secondary phone. Don't like my recommendation? Fine, recommend something else. Probably something that you have been used for some time

@800XL I can suggest Pixel 3a/3a XL, it works very well with Ubuntu Touch. Unfortunately, there is not so much apps yet for Ubuntu Touch just because you can't simply install any app like on regular Linux distro - it either should be listed on OpenStore, or at Ubuntu's 20.04 APT repos (in this case, it's possible to install them via Libertine). Snaps/flatpaks is not supported yet.
Ubuntu Touch itself is in a pretty good shape now. But the tradeoff is a lack of apps