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.
To Vanquish the Sea HagEdit
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