<p>You are given a string <code>num</code> representing a large integer. An integer is <strong>good</strong> if it meets the following conditions:</p>
<ul>
<li>It is a <strong>substring</strong> of <code>num</code> with length <code>3</code>.</li>
<li>It consists of only one unique digit.</li>
</ul>
<p>Return <em>the <strong>maximum good </strong>integer as a <strong>string</strong> or an empty string </em><code>""</code><em> if no such integer exists</em>.</p>
<p>Note:</p>
<ul>
<li>A <strong>substring</strong> is a contiguous sequence of characters within a string.</li>
<li>There may be <strong>leading zeroes</strong> in <code>num</code> or a good integer.</li>