When three hags of any type gather, they can form a coven to gain increased magical ability. Any combination of hags can form a coven, but green hags are the most common members of such foul gatherings.
Whenever all three hags of a particular coven are within 10 feet of one another, all three of them can work together to use any of the following spell-like abilities: animate dead, baleful polymorph (DC 18), blight (DC 17), bestow curse (DC 17), clairaudience/clairvoyance, charm monster (DC 17), commune, control weather, dream, forcecage, mind blank, mirage arcana (DC 18), reincarnate, speak with dead, veil (DC 19), vision.
All three hags must take a full-round action to take part in this form of cooperative magic. All coven spell-like abilities function at CL 9th (or at the highest CL available to the most powerful hag in the coven). The save DCs are Charisma-based, and function as if with a Charisma score of 16 unless one of the hags has a higher Charisma score, in which case the spell-like ability DCs are adjusted by that hag's Charisma modifier.
At the GM's discretion, certain more powerful hag covens might have additional spell-like abilities.