don't give up

The situation may not be that bad, even if you don't have a reference from B, you still have an excellent reference from A. One excellent reference is a lot more powerful than 3 fine references. Go back to your situation, as 老土建議 too, do you have a good freind in Company B? If you have one, you should ask him to be your reference and prepare him with a list of things that you want C to hear. Obviously C ideally wants to talk to your former supervisor, you may have one the following choices:
1. your friend was your supervisor
2. your former supervisor was no longer with the firm so your friend is the best person to comment on your work
3. your former supervisor is not willing to give reference due to firm policy but your friend is willing to do your favor to share his opinion with C
4. give a call to your former supervisor to see if he is willing to give you a good reference. If he doesn't want, he may tell you that it's due to firm policy, then that's probably want you want to hear...:-)
I don't think that any of the choices is far off the fact that you worked with B. Just do your best and don't worry too much.