.green { background-color : lime ! important }
p:not(:nth-last-of-type(3)) { background-color : red }
dl > *:not(:nth-last-of-type(3n+1)) { background-color : red }

<p>This paragraph should be unstyled.</p>
<address>This address should be unstyled.</address>
<p class="green">This paragraph should have green background</p>
<p class="green">This paragraph should have green background</p>
<dl>
  <dt class="green">First definition term that should have green background</dt>
    <dd class="green">First definition</dd>
  <dt class="green">Second definition term that should have green background</dt>
    <dd class="green">Second definition</dd>
  <dt>Third definition term</dt>
    <dd>Third definition</dd>
  <dt class="green">Fourth definition term that should have green background</dt>
    <dd class="green">Fourth definition</dd>
  <dt class="green">Fifth definition term that should have green background</dt>
    <dd class="green">Fifth definition</dd>
  <dt>Sixth definition term</dt>
    <dd>Sixth definition</dd>
</dl>