Meaning: The rain was falling heavily and steadily from the sky.
Choice of Words: Pour means to flow heavily and steadily. Down means from a higher to a lower place.
Meaning: This sentence also describes heavy rain falling from the sky, but it emphasizes the force of the rain.
Choice of Words: Coming means moving towards a place. Hard means with force or intensity.
Meaning: This sentence describes heavy rain falling from the sky, and it gives the impression that the raindrops are hitting the ground with force.
Choice of Words: Pelting means hitting or striking repeatedly.