I think 3 sword style Zoro is obviously stronger. Like some pointed out the reasons, even if it didn't made any sense, it doesn't have to. It's fiction and so generally thr more swords, Stronger Zoro is. If i am not wrong, his strongest attacks are 3 sword style based, like he did against Mihawk, Kuma, Kamazou or even Pica, he rely on 3 sword style for final attack, it's his signature moves or distinguish him from othet swordmans.