pluton blasts the red line away.
poseidon helps the evacuation of fishmen, controls the sea monsters and stops the whirlpool cause by red line bombing.
uranus will…..ummm...anyways....it could be anyone's guess.
In the world of One Piece, there are more than just land and sea formations, there are also a number of sky islands (as implied in Skypiea). If Poseidon helps keep the inhabitants of the sea safe, and Pluton unifies the land and sea, maybe Uranus unifies the ground and the air!
In ancient Greek/Roman mythology, Uranus was literally the sky and husband (or consort) of Gaea, who was literally the earth. It would make sense for that weapon to unify the heavens and the earth.
My question would be why Oda would choose to name the weapons Poseidon, Pluton, and Uranus, when it would be more logical to name them after the three sons of Kronus, son of Uranus. It would seem more obvious to name the third weapon Zeus or Jupiter, because he is the third brother, and is also considered a god of the skies.