<h3>Without Sidebar</h3>
<main>
  <section></section>
</main>
<h3>With right Sidebar</h3>
<main>
  <section></section>
  <aside></aside>
</main>
<h3>With left Sidebar</h3>
<main>
  <aside></aside>
  <section></section>
</main>

main {
  display: grid;
  grid-template-columns: 1fr;
  grid-auto-flow: column;
  height: 100px;
  gap: 20px;
  border: 5px solid;
}
section {
  background: red;
}
aside {
  width: 100px;
  background: blue;
}
aside:first-child {
  grid-column-end: -2;
}