From The Dhamma Encyclopedia
Sikkhā: the 'training', which the Buddha's disciple has to undergo, is 3-fold: training in higher morality adhisīla-sikkhā in higher mentality adhicitta-sikkhā and in higher understanding adhipaññā-sikkhā This 3-fold training refers to the 3-fold division of the 8-fold path magga in morality, concentration and understanding sīla samādhi, paññā In D. 16 and A.IV,1 it is said:
It is through not understanding, not penetrating noble morality... noble concentration... noble understanding... noble deliverance that I, as well as you, have had for such a long time to pass through this round of rebirths.
This then is morality, this concentration, this understanding, this deliverance. Being endowed with morality, concentration brings high fruit and blessing. Being endowed with concentration, understanding brings high fruit and blessing. Being endowed with understanding, the mind becomes freed from all fermentations āsava namely, from the sense-fermentation kāmāsava, from the fermentation of existence bhavasava from the fermentation of opinions ditthisava from the fermentation of ignorance avijjāsava.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.