three main sutras (of a school of Buddhism)
Refers to the three foundational sutras of a particular Buddhist school, such as the Pure Land school's three sutras.
The three main sutras of the Pure Land school are the Larger Sutra, the Contemplation Sutra, and the Amida Sutra.
From 三 (three) + 部 (section, part) + 経 (sutra), literally 'three-part sutra'.