Zoro has trained with Santoryu for basically his entire life, and has completely perfected it by this point. But, just from a fighting perspective, it would be quite difficult to get enough leverage/support with a sword in your teeth to actually do effective damage to anything (at least, without Haki). Unless this is just a classic case of "this is a fantasy world with fruits that give people magical powers, so it's possible for Zoro to have a strong enough neck to use a third sword effectively".
Most of the times we see him fighting, it's usually only with his two swords in his hands, and we barely ever actually see his third sword do anything. We often see implied strikes after he finished Oni Giri or other big finishing moves.
It seems Nitoryu would be just as effective as Santoryu in most situations. What exactly does his third sword do in a normal fight, that is so important for his style to hinge on it? Did the third sword become more useful in normal combat after the timeskip (not including the use of Haki, because if that was included, obviously it would be more useful), or is it still basically the same?