According to Newton's third law of motion, the reaction force exerted by the air on the balloon is equal in magnitude but opposite in direction to the action force exerted by the balloon on the air.
Given that the magnitude of the action force (F_balloon) is -3 N, the magnitude of the reaction force (F_air) will also be 3 N. The negative sign indicates that the forces are in opposite directions, but when considering magnitudes, we ignore the negative sign.
Therefore, the magnitude of the reaction force of the air pushing on the balloon is 3 N.

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