: Copy the file directly to the /switch/sonic3air/ folder on your SD card.
The file is the official ROM data for Sonic 3 & Knuckles Sonic Knuckles Wsonic3.bin File Download - Google
: If you own a standard "Sonic 3 & Knuckles" ROM (often named s3k.bin or sonic3k.bin ), you can simply rename it to Sonic_Knuckles_wSonic3.bin for compatibility with Sonic 3 A.I.R. setup. Usage in Sonic 3 A.I.R. : Copy the file directly to the /switch/sonic3air/