Three-Column Layout with CSS Grid

HTML
<div class="container">
  <header class="header">
    <p>header</p>
  </header>

  <!-- Replace <div> with <aside> if your content has one aside. For more than one, nest <aside> elements in the <div>. -->
  <div>
    <p>sidebar 1</p>
  </div>

  <main class="content">
    <p>main</p>
  </main>

  <!-- Ditto the previous note about <aside>. -->
  <div>
    <p>sidebar 2</p>
  </div>

  <footer class="footer">
    <p>footer</p>
  </footer>
</div>

CSS
/* Sections of Layout
-------------------------------------- */
/* Layout is stacked vertically by default (for narrower viewports), so give some breathing room between the sections. */
.container > * {
  margin-bottom: 10px;
}

/* Now let's apply grid for wider viewports. */
@media screen and (min-width: 40em) {
  .container > * {
    margin-bottom: 0;
  }
  /* Define the grid */
  .container {
    display: grid;
    grid-template-columns: 1fr 3fr 1fr;
    grid-gap: 10px 20px;
  }
  /* Place items on the grid */
  .header,
  .footer {
    grid-column: 1 / -1;
  }
  .content {
    grid-column: 2 / 3;
  }
}

/* Generic styles for demo purposes
-------------------------------------- */
.container {
  font-family: Helvetica, Arial, sans-serif;
  margin-left: auto;
  margin-right: auto;
  max-width: 75rem;
}

.container > * {
  background-color: #ccc;
  padding: 1em;
}

/* Typically, you wouldn't specify a height or min-height on this, instead allowing your actual content (i.e., text, images, etc.) to dictate the height of your content area. But since this example has very minimal content, I've set a min-height to mimic a taller content area. */
.content {
  min-height: 350px;
}