How are the characteristics of the Earth, the Moon, and the Sun similar and different?
They are spherical by their own gravity.
The Sun is made up of gases, most of them (inside) in plasma state, fueling a gigantic nuclear reaction.
The Earth is made up mostly of rock, with an inner core of iron and nickel. The inner part is in a liquid state (=hot).
The Moon is made up of rock only. All of it is in solid state.
And of course the sizes are quite different...