Dominus Trinus

The Dominus Trinus spell was located in the front of the Book of Shadows, after the woodcut of the three witches. Phoebe Halliwell unknowingly cast it, unbinding the powers of the her and her sisters; Prue and Piper, aswell as their collective power; The Power of Three.

The spell was located in the very front of the Book of Shadows, but it's unknown who wrote it. It is likely that Penelope Halliwell wrote it for her Prue, Piper and Phoebe at some point after she bound their powers.

The spell is presumably based on the spell "A Spell for Invoking the Power of Three" to invoke the powers of the sisters.

History
In 1998, after the Spirit Board told Phoebe to check out the attic of the Manor, she went there and found the Book of Shadows in the trunk that her grandmother had put it in seconds before her death. She found the spell shortly after opening the Book, and read it aloud. This invoked the powers of her and her sisters, which was its intention

Dominus Trinus

 * Hear now the words
 * of the witches,
 * The secrets we hid
 * in the night.
 * The oldest of Gods
 * are invoked here,
 * The great work of
 * Magic is sought.
 * In this night and
 * in this hour,
 * I call upon the
 * Ancient Power.
 * Bring your powers
 * to we sisters three!
 * We want the power!
 * Give us the power!