If the opponent takes 3 marbles, the first player should take 1, if the opponent takes 2 marbles, the first player should take 2, and finally if the opponent takes 1 marbles, the first player should take 3.
The first player should take 1. The opponent takes 3 marbles. The first player should take 2. The opponent takes 2 marbles. The first player should take 3. The opponent takes 1 marbles.