(1) When the hot air balloon ascends, its mass remains unchanged and its height increases, so its gravitational potential energy increases.

(2) After the hot air balloon is heated, the air inside the balloon expands, but the quality of the gas inside the balloon remains unchanged. According to the formula ρ=mv, the density of air in the balloon becomes smaller.

(3) The reason why the hot air balloon rises is that the air inside the balloon is heated and expanded, and its volume becomes larger. According to Archimedes principle, that is, F float =G row =m row g=ρ gas V row G, the buoyancy of the balloon becomes larger at this time. When the buoyancy of the balloon is greater than the gravity of the balloon, the hot air balloon can rise.

So the answer is: (1) gravity potential; (2) density; (3) After the balloon is heated, the air density in the balloon becomes smaller and the volume becomes larger, so the air buoyancy becomes larger. When the buoyancy of the balloon is greater than the gravity of the balloon, the hot air balloon can rise.