Authorship dispute after author was removed without their knowledge
You have described a situation very similar to the one described here—unfortunately, you and your colleagues have mishandled the situation and are at fault through lack of communication, even though it was an oversight.

Since you did not communicate with co-author C that you were going to remove him as an author, he is entirely within his rights to raise a complaint against you. If he can provide sufficient evidence that the editors at the journal or the administrators at the university decide that you did not treat him fairly, you can be subject to sanctions. The paper certainly could be retracted, and you could be subjected to administrative punishment from the university as a result.