It is never lawful to duck. If you cannot or do not wish to beat the card in front of you, you must take it up into your hand, and it is then the next player's turn to lead to a new trick. If there is more than one card lying on the table and you cannot beat the last card played, it is only this last card which you must take up into your hand. It is then the next player's turn to beat the card which was underneath it. In the second case, the player to the left of the player who picked up the last card leads to a new trick.
B sets aside the three cards and leads to the next trick.
A picks up his own card. It is now B's lead. Note that as players run out of cards, the number of cards in each trick reduces. The trick is now over, and C leads to the next trick. As there are now only two players, the new trick will have only two cards.