nth-child

• apple 1
• apple 2
• apple 3
• apple 4
• apple 5
• apple 6
• apple 7
• apple 8
• apple 9

html:

  <ul>
    <li>apple 1</li>
    <li>apple 2</li>
    <li>apple 3</li>
    <li>apple 4</li>
    <li>apple 5</li>
    <li>apple 6</li>
    <li>apple 7</li>
    <li>apple 8</li>
    <li>apple 9</li>
  </ul>
  

css:

    ul li:nth-child(3n+2) {
      color: red;
    }