The simple answer is to identify the most secure and compatible hashing algorithm supported by all three platforms, settle on this and alter the systems configurations to honor this algorithm and use it for future password generation. Then a method to generate the passwords for each user using the same salt on all three platforms could be devised along with a way to generate the users passwd/shadow entries and then a method to add these to password/shadow files on target systems.

All of this would be simplified to a great degree if central authentication was in use, ala LDAP or NIS, unless I'm misunderstanding. Otherwise it's a poor man's directory service