You have chosen that he should play the 3 of Spades. If he had played either of the other cards, he would have been doubling up and playing 2 winning cards on the same trick.
Now we have to consider what card should be played by East, to this first trick.
Here is East's hand.
He of course can only see his own hand, and the exposed dummy. He knows his partner led the 9 of Spades and the dummy has played the 3.
10. Which of his cards should he now play to the trick?