So the hags have a dastardly scheme underway to magically trap and corrupt an ancient silver dragon (which would turn the dragon's alignment from lawful good to chaotic evil). Dreadful Jenny abetted a black dragon's wickedness in order to obtain a black-magic orb that lets its user cast Meteor Swarm once per day. Wicked Nora stole a secret from the Evereskan archmage who was once mentor to the party's sorceress PC - and knew just when to steal a sentient (neutral evil) greatsword from the hoard of the silver dragon who is the coven's target. Cackling Vanya identified the exact...aasimar...inhabiting the world of Toril who made the ideal blood sacrifice, and thus sent a minion to kidnap her from a shrine to Eldath (to whom this aasimar is a cleric) hidden in the slums of Luskan.
The key pieces of imagery with which I've taunted the PCs leading up to this point are silver dragons that are missing a left wing, and the Seven of Swords card in the Tarokka (in-universe tarot). The trap consists of orb, greatsword, and intended blood sacrifice all inside a ring of seven one-winged dragon effigies made from woven branches (Huge size category, so they have formidable HP), and the PCs need to destroy five of these effigies in order to disable the hags' magic trap. (Echoing the image of the Seven of Swords card depicting a man running away from a military camp with five swords, while two more swords lie carelessly dropped on the ground.) In addition, of course, to their Shared Spellcasting coven feature, each hag has a Seven of Swords card of her own that lets her cast one 7th-level spell per day. The trap depends on one of the hags using her card to cast a well-timed Reverse Gravity, but the hags can also cast Crown of Stars (Xanathar's), Power Word Pain (also Xanathar's), or Prismatic Spray as they choose through their cards.
Because I want to encourage the PCs to set as much of the battlefield on fire as they possibly can without hurting the aasimar (a highly ironic reference to the disaster that they're trying to prevent), I'm thinking of having most of the hags' minions be scarecrows and flesh golems. Question is, though, how many of each would be about right? Should one of the hags conjure other minor minions mid-battle? Of course I'll equip each of the hags with a few poisoned darts or daggers, or flasks of acid, that they can throw at PCs clever enough to attempt battlefield control that separates one hag from her sisters.
One thing worth noting is that the aasimar is a Level 7 Peace Domain cleric, who will have no gear except Eldath's symbol embroidered into her clothes so that she has a spell focus, and be visibly roughed up due to mistreatment by Cackling Vanya and her minions. Per Eldath's pacifist dogma, she will do little to no actual fighting once one of the PCs frees her of her bonds - mostly dodging and distracting. At a critical point in the battle, however, the silver dragon who is the hags' target will fly in, flaring his Frightful Presence to see how the PCs react. (Sorceress, having silver dragon ancestry herself, alone has advantage on her WIS save. PCs who fail their saves may mistake the silver dragon for either Arauthator or Arveiaturace, white dragons to either of which the hags' trap site might be territory.) The aasimar will make the critical mistake of trying to drag the greatsword away from the hags' trap upon recognizing it as stolen dragon's treasure, and the greatsword will lash out with necrotic damage at the aasimar and any PCs who try to help her move it.