What do you call a 成语 made up of three words instead of four? What do you call a 成语 or expression made up of 3 words instead of 4? For example, 夜猫子, 绊脚石, or 避风头.
No surprise here, they are called 三字成语 (three-letter 成语).  However the words in your question are usually not considered 成语, they are 俗语.  三字成语 do exist, though, such as 破天荒, 莫须有, and 闭门羹.