Why are Mars, Moon and Venus unfit for human habitation?

Mars has a low atmospheric concentration, almost no oxygen, low temperature and no liquid water on its surface.

There is no atmosphere on the surface of the moon, and the temperature difference is too great. There is no water on the surface, but there is helium 3. As long as technology permits, mining robots are still valuable.

Venus's atmospheric concentration is too high, the temperature is extremely high, up to 460 degrees, and the environment is extremely bad. You can leave if you want.