त्रयाणां ब्रह्मविष्णुरुद्राणाम् अम्बकं पितरम् – ie., the father of Brahma, Vishnu & Rudra. [The Sayana Bhashya for the Rg.Veda mantra is attached. 59th सूक्तं of 7th मण्डलं from ऋग्वेदः .. The Rsi for this sUkta is मैत्रवरुणिः वसिष्ठः ]. [Thanks to Sri Venkata Sriram ji for the Sayana bhashya reference]
There are 12 Rks in this sUkta and the 12th (last) Rk is called मृत्युविमोचनी ऋक् ie., a verse which frees the devotee from death.
Though not directly related to ‘trayambaka’, for the name ‘Keśava’ in the Vishnu Sahasra nama – Shankaracharya, alternatively, has given an etymological meaning: कश्च अश्च ईशश्च त्रिमूर्तयः केशास्ते यद्वशेन वर्तन्ते स केशवः ।
[Brahmā, Vishnu and Shiva are the trimūrti-s – all of whom are under the control of that entity which is called Keśava.]
Just as Trayambaka is an entity ‘above’ the Trimurtis, Keshava too is ‘above’ the Trimurtis. This idea of the Supreme Brahman as transcending the Trimurtis is admitted by Kamban too. In Advaita this is known by the term ‘Turiya’. Turiya can be looked upon through any name such as Shiva (Para Shiva) and Vishnu (Keshava as determined by Shankara above). The Para Vishnu idea is found in the Vishnu Puranam too:
स संज्ञां याति भगवानेक एव जनार्दनः ॥ १,२.६६ ॥
स्त्रष्टा सृजति चात्मानं विष्णुः पाल्यं च पाति च ।
उपसंह्रियते चान्ते संहर्ता च स्वयं प्रभुः ॥ १,२.६७ ॥
