My view is that A & B are still trustees as B did not retire and C was not validly appointed. This has nightmarish possibilities if A and C have purported to exercise discretionary powers. They would be strictly liable for any losses.
It may be possible to resolve this with a deed of retirement of C and appointment ( by A and B) if backed with an opinion from suitably qualified counsel. If it is a heavy case or there are actual losses - e.g. if substantial appointments of capital have been made, then an application to the court would be safer.
Tim Gibbons