Often refers to Sonic the Hedgehog 3 (ROM hacks/mods) or Amazon S3 (cloud storage buckets).
If you are looking for the actual contents of a file you own with this name, here is what those characters typically suggest in various communities: S3SLC3.rar
Could stand for Single-Level Cell (a type of high-end flash memory) or a specific Location Code . Often refers to Sonic the Hedgehog 3 (ROM