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

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