Assume that the Rebuy will cost you at least 3 buyins, $9 in the case of your $3 tourney.
Rebuy at the first opportunity in order to give yourself a double-stack. Play tightish, but be prepared to see a flop from late position with anything promising, looking for an opportunity to hit the flop hard and double up.
If you are unlucky enough to be busted, it may be worth buying in again during the first half-hour only, but generally not after that. The average chip stack will already be so high that you will be at a disadvantage and would do better to move on to the next tournament.
At the end of the rebuy period, always take the add-on unless you are shortstacked and struggling (in which case, again, you might do better to save your money for the next game).
Your goal, in an ideal world, is to have 5x the starting stack at the end of the first hour - 1 buyin, 1 rebuy, doubled up, 1 addon. Then you are in a deepstack tourney, in effect. Play your normal game and good luck.