Improved Averaged Distribution of $d_3(n)$ in Prime Arithmetic Progressions

Abstract: We say that $d_3(n)$ has exponent of distribution $θ$ if, for all $\varepsilon>0$, the expected asymptotic holds uniformly for all moduli $q \le x^{θ-\varepsilon}$. Nguyen proved that, after averaging over reduced residue classes $a \bmod q$, the function $d_3(n)$ has exponent of distribution $2/3$, following earlier work of Banks et al. Using the Petrow--Young subconvexity bound for Dirichlet $L$-functions, we improve this to an exponent of distribution $8/11$ when averaging over residue classes modulo a prime $q$.