If you have not updated rvm do that first RVM Upgrading
rvm get stable
# or
rvm get master # for even newer versions not in stable 3.0.0 in this case
To see all available rubies run
rvm list remote all
# or
rvm list known # as pointed out in the comments
you should see ruby-3.0.0 in the list of available rubies
Then run
rvm install ruby-3.0.0