The term Sanpitsu (三筆) or "three brushes" is used in Japanese to refer to a group of three famous Heian period calligraphers:
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Later groups of calligraphers were named in imitation of the original Sanpitsu.
Kan'ei Sanpitsu (寛永三筆)
Named for the Kan'ei period in which they flourished:
Obaku no Sanpitsu (黄檗三筆)
Zen Chinese calligraphers who worked in Japan.
Bakumatsu no Sanpitsu (幕末の三筆)
Meiji no Sanpitsu (明治の三筆)
Shōwa no Sanpitsu (昭和の三筆)
References
Sanpitsu Wikipedia(Text) CC BY-SA