In case anyone else is wondering why Ruby 3 doesn’t show when you do asdf list-all ruby 
do this first:
asdf plugin-update ruby
Then you can install Ruby 3 with:
asdf install ruby 3.0.0
Or:
asdf install ruby latest
Also updated my blog post here.
Think it would be good if asdf list-all ruby simply does asdf plugin-update ruby beforehand - might save a bit of head scratching for a lot of people 