Problem 1. "A" opens. "B" stays with Q 8 4 3 A in different suits. "A" stands pat. "B" discards the Queen and draws a 6. "A" checks. What should "B" do?

Answer. "B" should bet. The chances are that "A" has a worse hand, probably a nine high. He will probably call even so, for players who have opened the pot are wary of a bluff in this situation.
Problem 2. "A" opens. "B" stays with Q 8 4 3 A in different suits. "C" also stays. Everybody draws one card, "B" getting a 6. "A" checks. What should "B" do?

Answer. "B" should also check. Against 2 one-card draws an 8 6 is not good enough for a bet. If "C" bets, "B" is good enough to call.
Problem 3. "A" opens. "B" stays. "C" stays with 10 9 6 3 A. "A" and "B" draw one card. What should "C" do?

Answer. "C" should draw two cards to the 6. His ten high has little chance of standing up against two one-card draws. Also, when the decision is at all close, it is generally right to play for a good hand that will inspire some confidence in the end betting.