First Answer
Partner is on lead against three notrump.
Partner leads the nine of spades (four, Jack, five). Say that you return the three of spades (six, eight, King). What’s going on in spades? Partner’s lead of the nine of spades marks declarer with the ten, and clearly declarer has the Ace as well. Perhaps declarer started with A10xx (partner having 98x) and perhaps A10652 (partner having 98 doubleton).
What about high card points? Declarer promised 20-21, you have 5, dummy has 10, so that leaves 4-5 for partner.
Next, declarer leads a club to the Ace, a club to the King (you discard the five of diamonds), and the Queen of clubs (you discard the six of diamonds, declarer follows, and partner follows). Clearly, declarer is about to cash three more club winners from the dummy. Your first discard will be the ten of diamonds.
What will be your other two discards? Two spades, one spade and one heart, or two hearts?