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.
The answer is in this post.
熟语 (Well known phrases) include:
惯用语(common phrase), usually contain three characters, for example: "打秋風"，"拍馬屁"
For the first two, I may call them 三字詞(詞語). For the last one, I would call it 俗語.
But definitely not 成語. 成語should have a story behind it. Usually from history, fair tales etc.
詞語 in brief means it's formed by words that can be used independently. While putting them all together make the phrase more descriptive. Different combination of words may be used for the same thing/situation in different eras or even different communities in China