<div class="container">
  <p class="column">1</p>
  <p class="column">2</p>
  <p class="column">3</p>
</div>

.container {
  display: grid;
  grid-template-columns: minmax(min-content, max-content) 1fr 1fr;
  gap: 10px;
}

/* etc: */

body {
  margin: 1.5em;
  font-size: 130%;
}

.column {
  background: #ccc;
  border: 1px solid #666;
  padding: 1em;
  margin-top: 0;
  margin-bottom: 0;
  font-family: monospace;
  text-align: center;
  /* "border-collapse" hack */
  margin-left: -1px;
  margin-top: -1px;
}

.column:first-child {
  background: #b3e888;
  border: 1px solid darkgreen;
}

.container {
  border: 5px solid black;
  padding: 1em;
  margin-bottom: 2em;  
}

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.