<p>one</p>
<p>two</p>
<p>three</p>
<p>белый</p>
<p>зеленый</p>
$text = preg_replace('#(<p>(.*?)three(.*?)</p>)#is', '', $text);
$text = '
<p>one</p>
<p>two</p>
<p>three</p>
';
$text = preg_replace('#<p>.*?three.*?</p>#i', '', $text);
echo $text;
$text = '
<p>one</p> <p>two</p> <p>three</p>
';
$text = preg_replace('#<p>(?:(?!</p>).)*?three.*?</p>#i', '', $text);
echo $text;