You've answered yourself:
During Fishman arc, it was told that there used to be three weapons –
Pluton, Uranus, and Poseidon.
Yes, they are all (ancient) weapons.
The first time an ancient weapon was mentioned was in Alabasta.
Crocodile asked King Cobra where he hid the Poneglyph containing information about Pluton, an ancient weapon.
Later in the show, we get more information about Pluton: It's a warship with the capabilities to conquer the world. The blueprints for this ship were passed down in a line of shipwrights for centuries, until they fell into Tom's hands - the legendary shipwright. Later they're burned by Franky to prevent the World Gov. (Spandam) from building the ship.
The only thing we know about Uranus is that it's an ancient weapon, nothing else.
We could assume its an aircraft or something similar, according to the other weapons.
- Poseidon = Mermaid Princess = Underwater
- Pluton = Ship = Surface
- Uranus = ? = Air?