<div id="container">
  <div>
    Item as wide as the content,<br/>
    but at most 300 pixels.
  </div>
  <div>
    Item with flexible width but a minimum of 200 pixels.
  </div>
  <div>
    Inflexible item of 150 pixels width.
  </div>
</div>
#container {
  display: grid;
  grid-template-columns: minmax(max-content, 300px) minmax(200px, 1fr) 150px;
  grid-gap: 5px;
  box-sizing: border-box;
  height: 200px;
  width: 100%;
  background-color: #8cffa0;
  padding: 10px;
}

#container > div {
  background-color: #8ca0ff;
  padding: 5px;
}
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