: Files with "pmfw" often refer to "Project M" (a Super Smash Bros. Brawl mod) or general "Power Management Firmware." The "3s" could denote a specific version or hardware revision.
Based on its name and format, here are the most likely possibilities: pmfw3s.7z
If you can tell me or what you were trying to download when you came across it, I can help you identify if it's safe and what it's for. : Files with "pmfw" often refer to "Project