@harvimaxic The initial deadline is March 15 each year, but amendments are flexible.
You can consider C company as the initial filer, while your submission is the amendment.
@harvimaxic Yes, you need to tell C about B, or hand over the gensenchoshuhyo of B to C, IF C has not filed your taxes yet.
If they already filed, you'll need to amend your tax return on your own. It's not that difficult, just takes patience.