They must be valid package names. That rules out 2 (“import my-django-app” would be a syntax error). PEP 8 says:
Modules should have short, all-lowercase names. Underscores can be used
in the module name if it improves readability. Python packages should
also have short, all-lowercase names, although the use of underscores is
discouraged.
So, 1 and 3 are both valid, but 3 would be the recommended approach.