In the Season 5 episodes "A Witch's Tail" of the original series, the Charmed Ones help a mermaid dodge the Sea Hag. Piper said that there was a Power of Three spell in the Book of Shadows, but it was never shown or used.
Note: Since no one has ever seen the spell, the one listed below was entirely made up.
Appendices[]
To Vanquish the Sea Hag[]
Evil witch that dwells within water
Shall now perish by the Three of Power
Elements four conjoin as one
Let this fowl being be officially gone