Sho no sansei (three saints of calligraphy) (書の三聖)

Sho no sansei are three masters of calligraphy.

Kukai
He is also one of San hitsu (the three famous ancient calligraphers).

SUGAWARA no Michizane
ONO no Tofu
He is also one of San seki (the three great brush traces).

[Original Japanese]