The ritual requires a triquetra to be drawn in chalk on the ground wherever the ritual is to be performed. It requires, at the least, a few candles and bowl of sage. The wax of a candle is also apparently required to be poured out onto the ground inside the triquetra.
The incantation must be repeated until the ritual is successful. The Charmed Ones must be in complete harmony with one another in order for the ritual to be successful. The sisters stand at the triquetra, holding out their hands. An orb of white, electrical-like energy manifests in their palms and stretches to unify the three of them. If there is any type of imbalance between the three sisters, then the ritual will be ineffective and the connection will be lost.
In "Needs to Know", Mel, Maggie, and Macy tried to use this ritual in an effort to restore the Power of Three, now that all three sisters had active powers. However, while a connection appeared between Mel and Maggie, the ritual failed and the connection dissipated before it reached Macy.
Macy theorized that it was her demonic powers displacing her witch powers and preventing her and her sisters from restoring the Power of Three. In an attempt to rid herself of them, she went to Abigael Jameson-Caine. Abigael was confused as to why Macy would want to lose her demonic powers, but helped Macy strip them after finding where the Ritual Dagger used for demonic power stripping was located, secretly using it as a way to steal Macy's powers for herself. But while Macy was now free of her demon powers, her witch powers were still to return, meaning the Power of Three was still unobtainable.
In "Sudden Death", the sisters once again attempted the ritual, as Macy had regained her witch powers. This time, due to the three having active witch powers, a better connection appeared between the sisters, but it still withered and faded before the Power of Three was restored.
Now Maggie reasoned that Abigael had done something to Macy to prevent the Power of Three from being restored, but when she confronted Abigael about it, the half-witch found the notion amusing, as she had done nothing, and snidely said the problem isn't her, but lies in the sisterhood itself.
Restituere potentia trium
Restore the Power of Three