I am working with a convolution sum of the form
$$ h(j) = \sum_{k=0}^2 f\!\big((j-k) \bmod 3\big)\, g(k), $$
where $f, g : \{0,1,2\} \to \mathbb{C}$. Because of the modulo $3$ structure in the index shift, this looks like a (discrete?) circular convolution of $f$ and $g$. How can I rigorously show that it is indeed a circular convolution? Also, is this sum generalizable to any modulo $n$?