The Man 3

'The Man 3 ' is a texture present in Sonic Adventure 1, specifically on its Dreamcast version. The texture only appears once in the game, during Tails' final confrontation with Dr. Eggman on the Eggwalker. This movie poster-like texture replaces the typical 'Chao in Space' poster; and features a man with dark skin, sunglasses, a classy suit, and a large moustache, below this man appear to be several unknown silhouettes, seven to be exact.

The Man 3 has received some attention by Dreamcast-Era fans, specially after the upload of the Chaos in Space animated short, and some have decided to look more deeply into it.

despite rumors, the alternate poster only appears in the Eggwalker fight because the arena uses an older version of the Casino area of Station Square, likely replaced with Chao in Space because it fit more into the Sonic world

Men in Black/Matrix Comparisons
Many have compared the movie to Will Smith and Tomy Lee Jarvis' hit 1997 film Men in Black, arguing that the man in the poster looks quite similar to Will Smith, and that the outfit he is wearing is incredibly similar, if not identical, to that of a MIB agent. The visuals of the poster also resemble the style used in some of Men in Black's promotional material (Intensely lit background with black silhouettes contrasting over it). The titles of the movie and the one in the poster are slightly similar too: 'The Man' and 'Men in Black'. If this were to be a nod to Men in Black though, then why is it specifically 'The Man 3'?

Men in Black 3 released in 2012, a full 14 years after Sonic Adventure's original Dreamcast release; Men in Black 2 released in 2002, and first announced in 1999, so the developers of Sonic Adventure 1 would have no knowledge of the intention of making sequels for Men in Black. Some theorize that the very texture itself is evidence for the Universal Sonic Theory, that centres around the fact that the Sonic Franchise has predicted several events in modern history.

Some also argue that the poster is, in some form, predicting the hit 1999 summer blockbuster 'The Matrix', arguing that the man's attire bears a similarity to those worn by the protagonists of the films and the background of the upper part of the poster resembles a gigantic darkened wall of blue code, a visual used in several parts of the film.

Alien Agents
In relation to the Men in Black comparisons, some say that the poster hints at the existence of the popular Men in Black conspiracy theory; this would line up with the testimonies of some Universal Sonic Theory believers, who claim that these Men in Black have been hiding the supernatural events surrounding the Sonic Franchise since the original game's release in 1991.