Person A of your OT3 is away from B and C for a while. During that time, B dies (from what you can decide). When A returns, C has to break the news to them. Angry and upset, A blames B’s death on C. What happens next is up to you.

http://otpprompts.tumblr.com/post/141805333844/person-a-of-your-ot3-is-away-from-b-and-c-for-a

I had been thinking A = Frost, B = Sparrow, C = Galen, but I actually think A = Ion, B = Rylan, C = Avery is a more interesting option.