You start with six top tricks: one spade, three hearts and two clubs. Therefore, you need three diamond tricks.
The important thing to recognize on this deal is that you need to keep East off lead. If, say, you win the opening lead with the ♥K, and play a diamond to dummy’s king, it’s possible East will win the ace. If that happens, he will surely play a spade through your tenace before you’re ready for him to do so. If the spade finesse loses and diamonds don’t behave, the contract will fail.
So the best way to tackle diamonds is to play a low one from dummy and insert the 10. Even if that loses to the ♥J, you’re safe because West can’t attack spades from his side of the table.
If you’re going to play diamonds this way, however, you should win the opening lead in dummy. It may seem wrong not to win the opening lead in the short hand, but keeping the dangerous opponent off lead is more important.