Jaiminīya Upaniṣad Brāhmaṇa 2.6.3

2.6.3

sa yadi brūyāt trīn ma āgāyeti prāṇa udgītha ity eva vidvāṁs trīn manasā dhyāyet
trayo hi prāṇo 'pāno vyānaḥ
trayo haivāsyājāyante /

If he should say: “Chant for me for three”, knowing that breath is the udgītha, he should mentally focus on three.
For breath, out-breath, and diffused breath are three.
Three indeed are born to him.