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

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