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

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