This is called a 'take away' game (like nim).

In this simple case, I'm assumeing that the person to take the last packet wins.

Then you want to leave your opponent with a multiple of 3 packets. If he takes one, you take two, if he takes two you take one. Clearly, you'll take the last packet. Ergo, go first, and take two pakets.

If you want your opponent to take the last packet, the strategy is similar, but you always want to leave your opponent with a number that is equivalent to 1 mod 3 (that is, a number that has a remainder of 1 when divided by 3). By chopping off 3's you will leave him with the last packet, so Go first, and take one packet, and then match his moves as above.