Install chruby and ruby-install from Homebrew for proper management of Ruby versions:
brew install chruby ruby-install
Then use ruby-install to fetch and install the latest stable Ruby:
ruby-install ruby-3
Then use chruby to select Ruby 3 as the current version and check that it works properly:
chruby ruby-3
ruby --version
Then simply run bundle install and bundle exec jekyll serve to
grab the dependencies and build & serve the website.