It is occasionally used as a shorthand for "Star Trek" (ST) or "Stick" (STK) related game assets or "mod-packs."

Small "ST3K" or "STK" tools often refer to "Service Tool Kits" or specific automation scripts. ST3K.rar

If you have already opened it in a safe environment (like a Sandbox), look for a .txt file inside to explain the contents. It is occasionally used as a shorthand for