Setting: not sure, Earth or close equivalent. Present or near future, not that much high-tech stuff in evidence.

Male leader has died, one of his wives is overseeing the succession.

Probably a "religious" leader, story would work better if it were a voluntary sacrifice rather than natural causes. Year king is probably pushing it, seven year king would be better, but better still would be in response to some crisis.

Thinking that there are three wives embodying priestess roles, Maiden, Mother, and Crone. First instinct was that Crone was the survivor.

More interesting if Maiden was the survivor and was virgin. She becomes the "Eldest" wife in the next marriage. If I choose this route for the story, has some interesting implications for succession. Could sneak in "the lady always chooses."

If the guy did go to a ritual sacrifice, within context it's logical that the Crone slew him.

Hmm, what happened to the other two wives? Were they slain as part of the sacrifice? Tie in with Diana, virgin huntress?

How does society react? If present day, there are three murders.

Police procedural with viewpoint character one of the cops?

Possibly overdone, especially if you consider The Wicker Man. This variation could work if the cop/viewpoint character was "destined" to be the new Mother (without her knowledge of course). Would really be a twist if the cop was a guy and didn't know he was about to become bisexual with a little nudge from a Goddess. Not sure I am comfortable with writing bisexuality though.

Could go for the cop-out and make the investigating cop the new holy whatchamacallit, knowing that he'll be chosen for his own sacrifice one day. Strikes me as derivative.

Hmm, open or not? Have to admit, the hidden option works better dramatically. Could be totally predictable and make it officially hidden but unofficially The Secret Chiefs or some such. Really don't like that though, but then the question is why is there this marriage in the first place.

Secondary question, is there another related marriage that may be three males and one female? What would the roles be? Does it have any impact on this story? Probably need to explore this if it is longer than a short story.

Why was the sacrifice necessary? How to communicate that?

What is this guy's purpose and why did the marriage exist in the first place?