Per TDA Rule, Once all betting is complete, in an all-in situation, EACH players hole cards are to be turned face up. It is not allowed for any player to muck their hole cards. The dealer determines the winning hand and that player receives the pot.
Based on your description, the cards of both player B and player C are live. It appears the decision of the floor was absolutely correct. Player A would only be awarded the pot if that hand was the best hand of the three.
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