Sharing a file between projects

Say that I've got several projects called P1, P2, and P3. Each project in a separate git repo. Each project has file foo.cfg. However, whenever I change foo.cfg in P1, I have to make the same change to the foo.cfg in P2 and P3.

They really are separate projects which ought to have their own git repos, I don't want to force a user checking out P1 to also check out P2 and P3.

But I also don't want to have to manually make sure that foo.cfg is synced among all three projects.

Any ideas here guys?

-Patrick