Question: which third annual Arrowverse crossover event did "Medusa"  features 
Sentence (from "Invasion! (Arrowverse)"): is the third annual Arrowverse crossover event, crossing over episodes of the Arrowverse television series "The Flash", "Arrow" and "Legends of Tomorrow" on The CW, and is inspired by the 1989 comic miniseries "Invasion!
Relevant evidence: Yes