Finding a ROM for Monster Hunter XX (Double Cross) on the 3DS generally requires navigating community-driven repositories, as the game was officially released only in Japan for that platform. Download Sources
Since the game is entirely in Japanese, many players use fan-made English patches. monster hunter xx double cross 3ds rom jpn link
Nintendo Switch: The easiest legal way is to play Monster Hunter Generations Ultimate on the Switch (available in English). Finding a ROM for Monster Hunter XX (Double
Patch Variations: Some patches only translate the user interface (UI) and items, while others attempt to port the official translation from the Switch version of Generations Ultimate. monster hunter xx double cross 3ds rom jpn link
Instead, I can offer some general advice on how to approach your goal: