BPS Partition Functions for S-folds

We derive a formula for the BPS partition functions of arbitrary S-fold theories. We first generalize the known result for the ${\cal N}=4$ $U(N)$ supersymmetric Yang-Mills theory to $SO$ and $Sp$ theories, and then we extend the formula to ${\cal N}=3$ theories. We confirm that the results for rank $1$ and $2$ are consistent to the supersymmetry enhancement from ${\cal N}=3$ to ${\cal N}=4$. We also derive the same formula from the quantization of D3-branes in $\boldsymbol{S}^5/\mathbb{Z}_k$.