The first three threads is the Brahma vrata (promise of the Brahmin) and is related to Bhu loka (earth plane). The second set of three threads is given after marriage and is related to the Bhuva loka (solar system) while the third set of three threads Deeksha (initiation)and is related to svarga loka (heavens).
Atri maharishi is one of the ten sons of Creator Brahma, created by just the will of the Almighty and therefore designated as a Maanasa-putras (mentally generated issues). There were ten of these. Atri's wife is Anasuyaa, a daughter of Kardama Prajapati. They had three sons: Datta, Durvasa and Soma.
Rama, the son of Dasaratha, visited Atri Maharishi's Ashram during his fourteen years of stay in the forest. It was Atri who showed the way to Dandakaranya forest to Rama, after showering his hospitality on him.
Atri Maharishi is considered to be one of the great discoverers of sacred Mantras of Hinduism. In his family line there were a few other seers of mantras namely: Shaavaashva, Avishtir, and Purvaatithi. There were also other great Rishis in that line: Mudgala, Uddaalaki, Shaakalaayani, Chaandogya, etc.
Atri-samhita and Atri-smriti are two works attributed to Atri.