'Avengers: Endgame' taught us something really important about 3D ticket sales

As theaters fill up for "Avengers: Endgame" this weekend, single seats and tickets to 3D showings are some of the only options for moviegoers who haven't purchased tickets yet. While there was resurgence of 3D films in the early 2000s, popularity for these movies has declined dramatically over the last two decades, particularly in the U.S.