Mega Man Legends 3, known as Rockman DASH 3 in Japan, was a video game set to be the sequel to Mega Man Legends 2 and the fourth game in the Mega Man Legends series. It was announced for the Nintendo 3DS on September 29, 2010, during a Nintendo press conference for the 3DS. The game was to be directed by Masakuza Eguchi, with Tatsuya Kitabayashi acting as producer. Reo Uratani composed the game's music. Mega Man Legends 3: Prototype Version, known (roughly) as "Rockman DASH 3: The Prologue! Showing you the Game's Underbelly Version" in Japan, was an early version of the game that would be available for download in the Nintendo 3DS's eShop. The prototype would have feature playable segments as Barrett and ten missions. It was originally planned to be available upon the launch of the 3DS eShop on June 6, but it was delayed to be improved. Initially, it was said by Capcom that if the Mega Man Legends 3 Prototype Version does not do well upon its release in the eShop, Mega Man Legends 3 may be cancelled, but the Prototype Version was cancelled along with the main game. On July 18, 2011, a message was posted in the Devroom announcing the discontinuation of the project. The reason for the cancellation was that "it was not felt that the Mega Man Legends 3 project met the required criteria". No further explanation was given, and the Japanese Devroom was closed on August 19.

The game's story would have continued right after the events of Mega Man Legends 2. Two new characters, Aero and Barrett, would join returning ones to help rescue the protagonist, Mega Man Volnutt, from Elysium. Chris Hoffman of Nintendo Power implied the finished game may have focused on a mysterious item known as the Klicke Lafonica and the aftermath of the Elder System reactivating at the end of Legends 2.