Question: which third annual Arrowverse crossover event did "Medusa"  features 
Sentence (from "Invasion! (Arrowverse)"): Events at the end of the "Supergirl" episode "Medusa" on November 28, 2016 marked the beginning of the crossover, which officially began on November 29 with "The Flash", continued on "Arrow" on November 30, and concluded on "Legends of Tomorrow" on December 1.
Relevant evidence: No