When did Thanos appear in what movie?

Thanos appeared at the very beginning in "Avengers III".

Avengers III:

The supreme overlord of the galaxy, Thanos (played by Josh Brolin), led a few capable men to loot the entire universe, just to destroy everyone. The Infinity Stones are set in a metal glove, which can destroy the entire galaxy at the snap of a finger.

In order to save the universe, Tony Stark (played by Robert Downey Jr.) and Steve Rogers (played by Chris Evans) need to put aside their past grudges, reorganize the Avengers, and cooperate with Spider-Man (Tom Holland), Doctor Strange (Benedict Cumberbatch), Guardians of the Galaxy, Black Panther (Chadwick Boseman) and the power of the people of Wakanda Fight together.

Extended information:

"Avengers 3: Infinity War" is a science fiction film produced by Marvel Films. The film is directed by Anthony Russo and Joe Russo, and stars Robert Downey Jr., Josh Brolin, Chris Evans, Chris Hemsworth, Scarlett Johansson, Mark Rudd Starring Farrow and others, it was released in the United States on April 27, 2018, and in mainland China on May 11, 2018.

The film is the third film in the "Avengers" series and the 19th film in the Marvel Cinematic Universe. The film is connected to the plot of "Thor: Ragnarok" and tells the story of revenge. The League of Legends and their superhero allies sacrifice everything to stop Thanos from destroying half of the universe.

Film shooting

The filming officially started on January 23, 2017. The directors, the Russo Brothers, have released some sporadic photos of preparations for filming in Atlanta on social networking sites, mainly on the Atlanta set. land, and will also go to London for filming.

The entire film "Avengers: Infinity War" was shot using IMAX cameras. When it was screened in an IMAX theater, the entire film was expanded to the IMAX 1.9:1 aspect ratio. Companies involved in the film's special effects production include Industrial Light & Magic, Weta Studio, Double Negative (DNEG), Method Studios, Framestore, Cinesite, Digital Domain, RISE, etc.

Reference materials:

Baidu Encyclopedia--Avengers III