<p>USC only requires one letter of recommendation, so you already have an “extra,” but yes, you can also submit a third one from your principal who likes you.
[Freshman</a> Standards - USC Undergraduate Admission](<a href=“http://www.usc.edu/admission/undergraduate/apply/fresh_standards.html]Freshman”>http://www.usc.edu/admission/undergraduate/apply/fresh_standards.html)</p>
<p>When my daughter applied, she submitted one counlselor rec and two teacher recs. (She was accepted.)</p>
<p>Good luck!</p>