simplest way to visit 3 subdirectories with a command?

[dmitrey]

hi all,
could you inform how to compose a py-file (for soft installation),
that will visit 3 subdirectories (eg subdir1, subdir2, subdir3) and
invoke a command "python setup.py install" in each subdirectory?
I know there should be a simple solution available in Python
documentation, but I have an awful lots of other things to be done, so
could someone write these several lines of code?

Thank you in advance, D.