Things to remember
1) Count partner’s points also when playing defense, using your hand, dummy and the range promised by declarer. That will help you to know what to expect from him.
2) 3rd hand rules:
a. In general, 3rd hand plays high if 1st and 2nd hand plays low.
b. With a high sequence – play the LOWEST card of this sequence (unlike 1st hand, which generally plays top of sequence).
c. Dummy completes the sequence – and you generally need to keep your honor to dummy’s honor.
Examples:
You are East. What should you play as the 3rd hand? You should play the K. 3rd hand plays high (Note that vs NT, your partner could have the ace and declarer the Q, so playing the K is the way to win all the tricks in that suit).
You are East. What should you play as the 3rd hand? You should play the Q. 3rd hand plays high, but the lowest card from a touching sequence.
You are East. What do you play as your 3rd hand if dummy plays low? You should play the 10. 3rd hand plays high, but as dummy has the Q and as 3rd hand plays low from a touching sequence you need to play the 10. It’s high enough to win the trick if your partner has the ace and likewise it’s high enough to pull out the declarer’s ace, if indeed he has it.
You are East. What should you play as 3rd hand? You should play the 10. There’s a good chance your partner has the Q (especially if he led low). Switch to another suit (assuming your 10 wins) and wait with your ace over dummy’s K, ready to capture it when it’s played.
3) You can also use attitude leads during play; when you lead a high card you suggest a switch (so if partner wins this trick, it indicates that you don’t want him to continue with this suit). If you play a low card, it encourages your partner to continue with this suit. So here, when you played the ♣8, you suggested to your partner to switch when he wins the ♣A. The obvious switch is Heart, the suit you led, playing though declarer’s hand. In another situation, you could’ve had ♣KJ82; leading with ♣2 would encourage your partner to continue with this suit.
4) You hold Hearts A10xxx and declarer wins partner’s J with his K:
How do you know where the Q is? Your partner could have it, playing J from QJx and not having the ♣A, would mean a Heart return sets. However, if your partner doesn’t have it, but has the ♣A, you need to play a Club to his ace and get a heart return from him. To solve this issue, use SMITH signals. This signal, at trick 2, encourages or discourages (by either you or your partner), regarding the suit which was led (assuming it’s not important to give count in that trick, which has priority): a LOW card (like the ♦2 given here by East) will DENY the Q (discourage West to play Heart), while a high Diamond will encourage this suit.