The "three crowns" that Rosalind attributes to Phoebe symbolize her triple aspects as the virgin huntress, the protector of childbirth, and the goddess of the moon. As the moon goes through its phases, it can be seen as a waxing, full, and waning moon, which might have inspired Shakespeare's use of the term "thrice-crowned."
Rosalind's description highlights Phoebe's celestial beauty, her role in guiding nocturnal activities, and her association with chastity and virginity. The phrase adds poetic depth to the play, evoking the romantic and mysterious atmosphere of moonlight and the night sky.