<style>
#container {
  display: grid;
  grid-template-columns: fit-content(300px) fit-content(300px) 1fr;
  grid-gap: 5px;
  box-sizing: border-box;
  height: 200px;
  width: 100%;
  background-color: #8cffa0;
  padding: 10px;
}

#container > div {
  background-color: #8ca0ff;
  padding: 5px;
}
</style><div id="container">
<div>Item as wide as the content.</div>
<div>
Item with more text in it. Because the contents of it are wider than the
maximum width, it is clamped at 300 pixels.
</div>
<div>Flexible item</div>
</div>