The three primary incarnations of Shakti
According to a tale in the Shiva Purana, preparations for the incarnation of the three major forms of Shakti, Parvati, Sita, and Radha, started a long time before they actually appeared on Earth. Interestingly, the preparations for these three incarnations had begun at around the same time.
The narration tells us that three self-realized sisters with divine ancestory, Mena, Dhanya, and Kalavati, were once attending a spiritual conference at an abode of Vishnu. Unfortunately, they acquired a curse from Sanat Kumara for their slight impoliteness towards the sage and his three brothers . The curse involved the birth of the sisters as human beings on Earth. But as soon as the sisters apologized, the sage, saturated with mercy, modified his own curse into boons that were extensive by any standard.
Sanat Kumara said, “Mena, the eldest sister, will become the spouse of the god of the Himalayas. She’ll eventually reach Shivaloka to reside with her daughter, a form of Para Shakti. Dhanya, following her birth on Earth, will get married to King Janaka. She will later reach Vaikuntha to live with her divine daughter, the spouse of Rama. And Kalavati, the youngest sister, will become the spouse of Vrishbhanu; she will eventually reach Goloka to reside with her daughter, the shakti of Krishna.”
Though Para Shakti may incarnate on Earth in different ages, it appears that the perfect scheduling and design of leelas in the universe does not take her any time.
 Sanat Kumara and his brothers, Sanaka, Sananda, and Sanatana, are the earliest sages to appear in the universe; they had originated from Lord Brahma’s mind.