Use the Lair Actions and Regional Effects
My group of five level 6 PCs withdrew after a gruelling battle with a coven of green hags due (it seems to me from the DM side of the screen) to the number of Awakened trees they ran into, and the Hallucinatory Terrain (coven spell) the hags cast each day which the party didn't discern as an illusion. (They were too busy fighting and did not do a great job of scouting before the encounter). They had previously encountered one or two of the hags in the swamp, but each one had fled/withdrawn before the party could close on them.
I had, as I designed the coven and how they fit into the world, subbed in the Hallucinatory Terrain spell for the Grasping Vine spell since the Green Hag coven had taken over a portion of a swamp and that fit (DM side) thematically. What looked like a thick, good sized grove of trees was an illusion that masked their lair in a small copse of trees, some of which were awakened.
The hags were extremely reluctant to expose themselves, used their eye for scouting, and as another distraction there were a few swarms of ravens to keep the PCs busy. The illusory hag is the only one the PCs saw during the encounter.
The hag creates an illusory duplicate of herself, which appears in its
own space. As long as she can see her duplicate, the hag can move it a
distance equal to her walking speed as well as make the Illusion speak
on her turn (no Action required). The Illusion has the same Statistics
as the hag but can’t take Actions or Reactions. It can interact with
its Environment and even pick up and hold real Objects. The Illusion
seems real in every way but disappears if it takes any amount of
damage. Otherwise, it lasts until the hag dismisses it (no Action
required) or can no longer see it. (Volo's Guide to Monsters, Coven)
I used swarm of ravens as the random hag minions rather than swarm of rats or swarm of insects for thematic purposes. Insect swarms would also have sufficed. From Volo's Guide to Monsters:
- A powerful Green Hag creates one or more of the following additional regional Effects within 1 mile of her lair:
- Illusory duplicates of the hag appear in random places at random times (but never more than one in any given location). An illusory duplicate has no substance, but it looks, Sounds, and moves like the hag. The hag can sense when one or more Creatures are within 60 feet of her duplicate and can interact with them as if she were present and standing in the duplicate’s space. If the illusory duplicate takes any damage, it disappears.
- The region takes twice as long as normal to traverse, since the Plants grow thick and twisted, and the swamps are thick with reeking mud.
- Trees transform into awakened trees and Attack when Hostile intruders are near.
You can dial up, or dial down, the number of awakened trees/swarms to fit the three PC level 8 party; I'd suggest making the encounter between deadly and 2x deadly if you want this to be a big showdown.
Alternative Approach: consider what the hags might want from the PCs, or what the PCs might offer the hags, and figure out through play if they'll agree to a deal as a part of a negotiation - if the PCs try to parley with them. If successful, and the hags stop {doing whatever it is that has the PCs on their trail} an award of full XP would be a fitting wrap up to the encounter. The other group I DM for got a sizeable XP award for a recent negotiation with an Efreet that did not end up in combat, but instead in them making a deal.
I had not considered using a shared HP pool, but instead their tactics were based on "not being hit" and "using illusion and minions to make targeting them difficult".
Full disclosure: this is going to spoil what happened for both @V2Blast and @MikeQ (the hags remained unfinished business as other threats to the region arose) but I can live with that since they are now off on another adventure and are level 9. I suspect that if they head back to Saltmarsh at any point in the future, Granny Vine and her coven will be toast or will be forced to negotiate a deal.
