I think in the situation you describe, object C will be committed, and new objects A and B will be auto-committed - that is they are stored in memory until the end of your session. When you log out of the application, these auto-committed objects are destroyed.
If you wish to retain the new objects A and B as well as C, use an action button on C to replace the default save button. In the microflow attached to this save action button, commit all 3 objects before you close the page.
David are you sure about this? I think you are wrong. Object C does not own any of the relationships. So committing C does not implicitly commit the relationship, and therefore there would be no need for anything to be autocommitted here.
Also, autocommitted does NOT mean they are stored in memory, they are stored in database, but wil be deleted once the session is killed, unless they are explicitly committed first.