Spoiler for I probably misunderstoodA sees two 3's. Everyone knows 242 doesn't work.
Spoiler for It's a bit tricky...Right... we know that everyone sees a 3, but we only know that as the readers of the puzzle. C does not know that A sees two 3's, because C thinks he might have a 2 on his forehead. And if C did have a 2, then B could not know that A sees any 3's at all, since B might have a 4 on his forehead. So, since C doesn't know his own number yet, there is no reason for him to expect (nor is there reason for C to expect that B would expect) that A could deduce that his number was 3. The hard part is keeping track of who knows what in each circumstance.Spoiler for amount of 3'sA sees two 3's he knows B and C see at least one. That works for all people.
C knows that A sees at least one 3 (since B has one).
B knows that A sees at least one 3 (since C has one).
However, C does not know that B knows that A sees at least one 3, because to know that, C would have to (already) know that his own number was a 3.
Therefore, C cannot expect B to infer his number from the fact that A was unable to deduce his (at least on the first time around...).