Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.3.20

3.1.3.20

atho api trīṇi syuḥ | eko hyevāyam pavate so 'yam puruṣe 'ntaḥ praviṣṭastredhāvihitaḥ prāṇa udāno vyāna iti tadetasyaivānu mātrāṃ tasmāttrīṇi syuḥ

Or there may be three (stalks); for one, indeed, is that blower, but on entering into man he becomes threefold, to wit, the out-breathing, the in-breathing, and the through-breathing, and in accordance with his measure is this (cleanser): hence there may be three (stalks).