Shatapatha Brahmana 6.3.1.27

6.3.1.27

trivṛto bhavanti | trivṛddhyagniraśvābhidhānīkṛtā bhavanti sarvato vā aśvābhidhānī mukham pariśete sarvato yonirgarbham pariśete yonirūpametatkriyate

They (the halters) are triple (strings), for Agni is threefold. They are made like a horse's halter, for the horse's halter lies all round the mouth, and the womb lies all round the child: thus it is made like the womb.