貞宗三哲 Sadamune San Tetsu

What is Sadamune San Tetsu in Japanese Katana terminology?

Sadamune San Tetsu refers to the three outstanding swordsmiths who were disciples of "Sadamune", a representative disciple of "Masamune". The three are "Nobukuni of Yamashiro Province", "Motoshige", and "Houjouji Kunimitsu".