<p>A whole lot of scores of 1 would hurt in much the same way that a whole lot of grades of D would hurt, but the OP was asking about one grade of 3, and that’s a very different issue. If the OP has other competitive grades and scores, there is no reason to worry about a grade of 3 on an AP test (one that must have been taken in junior year or earlier, for this question to even come up) in the overall context of the OP’s application. </p>
<p>Yes, now that the Common Application, used by HUNDREDS of colleges, has spaces for self-reporting AP (and IB) test scores, college admission committees will see many more sets of scores and will soon learn that not all applicants ace all AP tests. Big deal. Don’t worry about it.</p>