To be honest, we don't know Mifune true strength because he was only shown stopping Hanzo from using jutsus and no using any of his own.What we do know is he's very skill, and quite fast but besides that nothing at all.From what little we know about Mifune I can put him at elite jonin to low Kage, but that is subject to change if kishi shows us something more from him.Also, the abc logic of Mifune being as stronger than three sannin doesn't work seeing as the sannin weren't at their strongest, and Mifune had intel on Hanzo from fighting him previously so he new how to counter his abilities.If mifune hadn't been previously cured by Hanzo poison he would have died from it in their last battle.Mifune knew to stop Hanzo from using his ninjutsu because Mifune have seen it before and knew of Hanzo strength.I doubt the sannin ever fought Hanzo before the time they lost against him.