  In this paper, we study non-hyperelliptic curves of genus $3$ with cyclic
automorphism group of order $6$. Over an algebraically closed field $K$ of
characteristic $\neq 2,3$, such curves are written as plane quartics $C_r: x^3
z + y^4 + r y^2 z^2 + z^4 = 0$ with one parameter $r$. As the first main
theorem, we show that $r\neq 0,\pm 2$ and give a necessary and sufficient
condition with respect to $r$ and $r'$ such that $C_r \cong C_{r'}$. By
describing the Hasse-Witt matrix of $C_r$ in terms of a certain Gauss'
hypergeometric series, we obtain the second main theorem, where we determine
the possible $a$-number of $C_r$, and give the exact number of isomorphism
classes over $K$ of such curves attaining the possible maximal $a$-number.
