Averages of coefficients of a class of degree 3 L-functions

In this note, we give a detailed proof of an asymptotic for averages of coefficients of a class of degree three L-functions which can be factorized as a product of a degree one and a degree two L-functions. We emphasize that we can break the 1/2-barrier in the error term, and we get an explicit exponent. Our proof relies on methods from the theory of exponential sums.