.red { background-color : red }
p:nth-of-type(3) { background-color : lime }
dl > :nth-of-type(3n+1) { background-color : lime }

<p>This paragraph is here only to fill space in the DOM</p>
<address>And this address too..</address>
<p>So does this paragraph !</p>
<p class="red">But this one should have green background</p>
<dl>
  <dt class="red">First definition term that should have green background</dt>
    <dd class="red">First definition that should have green background</dd>
  <dt>Second definition term</dt>
    <dd>Second definition</dd>
  <dt>Third definition term</dt>
    <dd>Third definition</dd>
  <dt class="red">Fourth definition term that should have green background</dt>
    <dd class="red">Fourth definition that should have green background</dd>
  <dt>Fifth definition term</dt>
    <dd>Fifth definition</dd>
  <dt>Sixth definition term</dt>
    <dd>Sixth definition</dd>
</dl>