E.B. Elliott in Horae revealed that the three frogs symbol came from an Egyptian medal. This would have to be a symbolic representation of their triple frog goddess Hecate.
The vial judgments are patterned after the plagues in Exodus. It is not a one to one correspondence but, the sores of the first vial, the blood of the second, the darkness of the fifth and the hail of the seventh vial establish the pattern. After the sixth vial there were three frogs.
The plagues were directed against the gods of Egypt. The frog goddess was one of their principle deities. Hecate was a triple goddess. She performed her magic at crossroads where three ways meet. As a result, the goddess of crossroads has three faces, one looking each way. She is also represented as an equilateral triangle. The Merovingian frogs are arranged in a triangle.
In paganism, the moon has three phases: crescent waxing, the virgin, full moon, the maid and crescent waning, the old maid. The word hag is a variant of Heka from Hecate. She was the goddess of witchcraft and Halloween. When you see the witch on her broom and black hat before a waning crescent moon you are seeing a representation of this egyptian goddess.