Why do people think Zoro's attacks with more swords are superior to those with just one?
With only one sword Zoro is able to focus all his strength on a single point, it is not surprising that the attacks were able to cut Mr 1, Ryumma and Kuma were of the Ittoryu style. In the fight against Kaku he used several swords as he needed to attack and defend himself at the same time.
I don't know if it was clear what I meant, my English is bad.
For those reasons:
1- In terms of slashes: 1 sword (360 pound canon) 2 swords (720) 3 swords (1080 pound canon)
2- In terms of techniques: His Sanzen Sekai (3 swords) > Shishi Son Son (1 sword)
And lastly, going into (Asura mode) involvesd 3 swords in order to create the 9 sword style...
And LASTLY, he's a 3 sword style swordsman (always known for santoryo).... Meaning, he excels BEST when he uses Santoryu... So, he can use two-sword style, he can use 1 sword style, but HIS BEST is 3 sword style
There can be an attack in 1 sword style stronger than 2 swords attack or EVEN 3 sword style... However, 3 sword style HAS the strongest technique in Zoro's arsenal