Imagine how powerful a Rooftop Sanzen Sekai would have been.
In all seriousness, it's obvious Zoro is a stronger fighter when using three swords, both because he's more versatile and because he has stronger moves, even if he has very powerful finishers and techniques with less swords. It's been a plotpoint spelled to the readers since as soon as the Usopp arc. It's not simply because "more swords = more strength", it's just the style he excels at the most.
Even the infamous shishi sonson has a stronger version of itself in rashomon.
And then there's the 9 sword style.