There is a prior discussion of this topic somewhere, but I don't have the time to look it up. Basically, my recollection of that discussion is that the statement "I'll put you all in" is an invalid, meaningless statement. I believe the correct action for your circumstance would have been for the dealer to stop the action immediately and require player B to clarify his action by stating a valid amount. Had he done that, player C would not have his problem.
That said, if player B had pushed out a stack of chips prior to player C acting, I would hold B to the amount pushed out whether anyone is all in or not. If player B had not pushed out chips prior to player C acting, I would consider player C to have acted out of turn and proceeded accordingly.
By the way, if this kind of thing is common by player B, I think you need to consider penalties for repeat offenders.
Chet