Three yanas of vedic asceticism

Three Inner Yanas of Vedic Asceticism (Tib. dka' thub rig byed kyi theg pa gsum) - These are the three outer classes of tantra: the yana of kriya tantra, the yana of charya tantra and the yana of yoga tantra.

You might wonder why are these called ‘vehicles of Vedic ascetism.’ It is because the three outer classes of tantra stress aspects of ascetic conduct, such as ritual purification and cleanliness, and in this respect they are similar to the Vedic tradition of the brahmins.