It contains a segment of the game's data. You cannot install or run the game using only "part3.rar"; you must have all parts (e.g., part1 through partX) in the same folder to extract them.
After extracting, look for suspicious .exe or .scr files that don't belong in a game folder.
Old versions of WinRAR or 7-Zip have known security flaws that can be exploited by malicious archives.