the 'training', which the Buddha's disciple has to undergo, is 3-fold:
training in higher morality (adhisīla-sikkhā)
in higher mentality (adhicitta-sikkhā)
in higher wisdom (adhipaññā-sikkhā)
This 3-fold training refers to the 3-fold division of the the 8-fold Path (magga) in morality, concentration and wisdom (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 wisdom ... 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 wisdom, this deliverance. Being endowed with morality, concentration brings high fruit and blessing. Being endowed with concentration, wisdom brings high fruit and blessing. Being endowed with wisdom, the mind becomes freed from all cankers (āsava) namely, from the sensuous canker (kāmāsava), from the canker of existence (bhavasava) from the canker of opinions (ditthisava) from the canker of ignorance (avijjāsava).