Poseidon, Pluton, and Uranus?
This is the text to stop people from being spoiled cause I would've killed someone if they would've spoiled parts of OP for me.
So recently we found out that Shirahoshi I believe is Poseidon, and has the power to control sea kings. And we already know that Pluton is a legendary warship. From the CP9 arc. Now we are possibly set up for figuring out what Uranus is. It was also said that Noah was a ship that Joy Boy. A former Poseidon. Was able to use. Now this got me thinking. Noah is a ship that is so far vastly bigger than any other ship we've seen, unless you count Thriller Bark as one, but it is more of an island. Pluton is known to be a legendary war ship.
What if Noah and Pluton are one and the same. It was made during the Void Century, so no one really knows it's origins. It is made to be pulled by sea kings, showing it's power as a battleship. Even a ship with Kairoseki wouldn't be able to avoid it and them. And this leads to my final thought, that the three weapons are meant to be used together. Imagine Poseidon, on a legendary warship surrounded by sea kings plus another power, all three united would be unstoppable, I don't care how many DF Blackbeard has, he would be killed by that no matter what. Who knows what Uranus is, maybe a protection thing. Idk, but no matter what, if the three are combined, it would be the supreme ruler of the sea.