At this point at least imo it seems unlikely that zoro is getting nidai or shodai kitetsu. So keeping that in mind it’s probably the case where zoro does just keep sandai and his current swords might be his final
I could be wrong but I don’t think he’s going back to wano to get nidai and I don’t he’d take shodai any time soon and master it
Back to sandai specifically we know that it was forged by sukiyaki aka oden’s dad. Even tho it was made by someone in wano it still made its way all the way to the east blue somehow
Maybe it was a sword that was given to Zoro’s grandmother passed down to his father and lost in his death. It somehow finding its place to ipponmatsu waiting for zoro
As for how the blade stacks up to wado and enma 2 great grade blades do you think it can ever be given a power up to catch up?
Like what if zoro feeds it a devil fruit or even makes sandai alone a black blade so it becomes the same grade as the other 2