Gayatri Mantra is a highly revered mantra from Rig Veda. The prefix "oṃ bhūr bhuvaḥ svaḥ" is known as "mahāvyāhṛt" (mystical utterance) is mentioned later in Taittiriya Aranyaka, which states to chant the mantra with this prefix.
Did the Sapta Rishis chant the mantra with this prefix?
It is very difficult to know how did the Saptarishis chanted the mantra, because they existed in different times. Before the revelation of the mantra to Vishwamitra they would have chanted Indra or other Surya mantra.
Regarding Vyâhritis, our grihya sutras prescribe addition of Vyâhritis before the Savitri/Gayatri mantra. Hence we can believe that it is a genuine practice. However, while chanting the samhita in various ways there is no need to utter the vyahritis. But in separate mantra anushthana or rituals it should be chanted.
The Vyâhritis preceded by (the syllable) Om (stand first); (these) and the Sâvitrî he should repeat three times and then recite the beginning of the Veda. Ashvalayana Grihya sutra 3.5.12
Having offered the chief oblations (belonging to each sacrifice) according to prescription, he adds the following oblations, viz. the Gaya, Abhyâtâna, Râshtrabhrit oblations, the oblation to Pragâpati, the Vyâhritis one by one, the oblation to (Agni) Svishtakrit with (the following formula), 'What I have done too much in this ceremony, or what I have done here too little, all that may Agni Svishtakrit, he who knows, make well sacrificed and well offered. Svâhâ!'
Having made oblations with the single Vyâhritis and with (the three Vyâhritis together), and having made an oblation with the verse, 'Thou art quick, Agni, and free from imprecation. Verily (satyam) thou art quick. Held by us in our quick mind (manas), with thy quick (mind) thou carriest the offering (to the gods). Being quick bestow medicine on us! Svâhâ!'—this (last) oblation contains an allusion to the mind (manas), it refers to Pragâpati, and alludes to the number seven (?),—he quickly repeats in his mind the dasahotri formula (Taitt. Ârany. III, 1, 1). Then he makes the sagraha oblation (?); (then follow the oblations), 'This, O Varuna' (&c.; see I, 2, 8, 16, down to the end of the Sûtra). Then he serves food to the Brâhmanas and causes them to say, 'An auspicious day! Hail! Good luck!' he then performs in the known way the sacrifice of a mess of cooked food to Agni.
He goes to a place near which water is, puts wood on the fire, performs the rites down to the oblations made with the Vyâhritis, and puts a piece of Palâsa wood on (the fire) with (the verse), 'Let us prepare this song like a chariot, for Gâtavedas who deserves it, with our prayer. For his foresight in this assembly is a bliss to us. Agni! Dwelling in thy friendship may we not suffer harm. Svâhâ!'
- Then he sacrifices with the Vyâhritis as above,
