In example 1 you are in a No Trump contract and the opening lead is the Jack of the suit shown. You play small from Dummy and East 
(your RHO) plays the King. Now for the counter intuitive play: unblock by playing the Queen! The opening leader is marked with the 10 of the suit. So playing the Queen lets you finesse Dummy's nine so you can have an Entry to Dummy and two tricks in the suit.

Do not be hoggish in example 2. Again you are in No Trump contract and the opening lead is the 4 of the suit shown. You play the 3 from Dummy which is followed by your RHO's 9. The moment of the unblock is upon you. If reflexively and without thinking you win the trick with the Jack then you just destroyed a valuable Entry to Dummy. Win this trick with the Ace. It actually does not cost anything because you will always lose one trick in the suit to the King. But winning the first trick with the Ace guarantees an entry to Dummy; either Jack to the Queen or the 5 to the 10.

In example 3 the only chance to make your No Trump contract is to get to Dummy.  There are positively no Entries with the three suits not show. The opening lead in the suit shown is the 5. You play the 4 from Dummy and your RHO puts down the Ace. Here you just got to step out of the box and make the spectacular unblocking play of the King! That is correct, throw the King under that Ace. Ignore the painful troubled stare of your partner and opponent's glee. (Glee in Bridge usually doesn't last very long.) Unblocking with the King establishes the Jack as a potential Entry to Dummy.