Legend
The story goes that Mahasadhvi Anasuya's (Anusuya) fame as chaste woman nonpareil spread to the worlds above. The Trimurthinis hearing it felt floored as they were treated inferior to mortals; a degradation that cannot be stomached; much less heard as the talk gained reverence from one and all.
Bitten by the jealousy bug, they brought their heads together to put an end through a strategy to her powers, like purging of the sins of pavana Gangamata by sprinkling of water, commanding the movements of planets with a mere utterance, boiling the pieces of steel grains into edible luscious tiny soft flakes etc. They requested their lords to dislodge her from the pedestal by depriving her chastity. Somehow! What a descent! How powerful art thou 0 power of jealousy grown too powerful to overpowering the super powers! The loyal Trimurthis set out at once on the sacred mission of polluting chaste Anasuya. Somehow!!
They changed themselves into brahmins and stood near the portal of Atri's hermitage with bowls, uttering Bikshandehi. Ah! fate what sports could thou demonstrate! Givers turned beggars!! somehow!!!
Due to the prevalent custom, Sadhvi Anasuya invited them for food. Just before serving, they with one voice laid a dishonorable condition -she should serve them naked. Woeful! Awfully woeful!! Being the gem of a women of impeccable chastity approaching undressed she sprinkled her lord's Padodakam!
Look! the Trimurthis turned into toddlers and started crying. She suckled them each with milk of divine knowledge. Atri was surprised to finding the infants playing on the lap of his wife. Seeing all what happened with his inner eye; he felt elated when the Vanquished Trimurtinis appealed to the mercy of Matrumurthi Anasuya for Patibiksha.
She restored the three to their original form. Before disappearing, the Trio blessed the couple with a son, partaking their amsas represented by three heads. And he was called Dattatreya and fulfilled the purpose of his incarnation -building of a new mission of Dharma on the ruins of ruinous sampradayas prevalent then.
Though this account differs with the first one, the ultimate end fulfilled Lord Mahavishnu's birth in the world. Some other legends have many anecdotes, but they converge at this point.
According to another legend, Atri performed a great tapascharya on Kula Mountain that set the whole world on fire. Brahma, Vishnu and Mahesha were impressed with him and granted him a boon. Atri asked them to be born as his children.