No premiere date announced yet. This announcement follows the conclusion of season 2 this week. Check out the AC article for additional information. Synopsis for the source manga from AniList:

One hundred and forty years ago in Kyoto, with the coming of the American “Black Ships,” there arose a warrior who, felling men with his bloodstained blade, gained the name Hitokiri, manslayer! His killer blade helped close the turbulent Bakumatsu era and slashed open the progressive age known as Meiji. Then he vanished, and with the flow of years, became legend.