Ōbaku no Sanpitsu (黄檗三筆) is a name given to a group of three famous Chinese calligraphers who lived in Japan:
They are all connected with the Ōbaku school of Zen Buddhism. Analogous groups of famous calligraphers include the Sanseki and Sanpitsu.
References
Ōbaku no Sanpitsu Wikipedia(Text) CC BY-SA