Hi,
I use OSX Mojave (10.14.6) which I can't update. I was prompted to update Joplin to version 3.0.15 but couldn't install it. I can't find the full details for this version in your change log, so I wanted to check if you've dropped support for myOS?
From looking at the release notes it appears that with Joplin v3.0.0, Joplin was moved from Electron v26 to v29. That "jump" included the changes made in Electron v27 where macOS 10.13 & 10.14 were no longer supported by the Electron maintainers. This was because Chromium had dropped support for those older macOS versions.