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

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