Newton's 3rd law will indeed hold if the experiment is done on the moon. The balloon will accelerate as a reaction to the escaping air, thus: the balloon exerts a force on the air when it is being released and, at the same time, the air exerts a reaction on the balloon. You can perform the following experiment (ask an adult for help): stand on a skating board (or put on a pair of skates) on a smooth, level surface.While standing still, throw a baseball in front of you. You will notice small recoil backwards (this is where an adult can help, so you don't fall down). You don't have to throw it hard; a gentle toss should do. When you throw the ball, you exert a force on it. This force is directed forward. At the same time, the ball exerts a reaction on your arm that pushes you backward. Notice the opposite directions between the force applied by your arm, and the reaction exerted by the ball. Next, try comparing the reaction when using a lighter ball and then a heavier ball. Have fun.