Let K be an algebraically closed field. In this paper, we consider the class of smooth plane curves of degree n + 1 > 3 over K, containing three points, P1, P2, and P3, such that nP1 + P2, nP2 + P3, and nP3 + P1 are divisors cut out by three distinct lines. For such curves, we determine the dimension of certain special divisors supported on {P1, P2, P3}, as well as an explicit description of all pure gaps at any subset of {P1, P2, P3}. When K = Fq, this class of curves, which includes the Hermitian curve, is used to construct algebraic geometry codes having minimum distance better than the Goppa bound.