I would solve for (H^+) of the HCl and add that to the first proton from H2SO4. You can check to see if HCN contributes enough to consider adding it in by setting up an ICE chart for HCN and using the sum of H^+ from H2SO4 and HCl as a common ion. I didn't do the calculation but I don't think the contribution from HCN is significant.
