mj-column-width.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column width="25%">
        <!-- Your first column -->
      </mj-column>
      <mj-column width="50%">
        <!-- Your second column -->
      </mj-column>
      <mj-column width="25%">
        <!-- Your third column -->
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>