[jh3]

project_root/sites/example.com

project_root/sites/example.com.a

project_root/sites/example.com.b

project_root/sites/example.com.a.123

project_root/sites/example.com.a.234

Create these directories. Add a settings.php file to each. Add stuff to sites.php if needed. example.com.a and example.com.b should definitely work. Not too sure about the other two because I haven't created a site in a multi-site setup that is 2 subdirs deep, but it is easy enough to try.

You may also need to symlink the subdirectories (e.g. a) in project_root. See "Subdirectory multi-site" here: https://www.drupal.org/documentation/install/multi-site