Well, look at the posted C41 developer formulas and then put in some sulfite to match that in the formulas. I would test part A and part B for sulfite by adding some acid to it and sniffing. It would only take a few drops of the concentrates and a few drops of acid. If you smell sulfur dioxide, then there is some on those parts. If there is, use less in part C.
Here is an example. Lets say that you need 10 g/l total Sodium Sulfite. If you smell no SO2 odor in parts A and B when acid is added,, then add 10 g/l to the final formula. Adjust the pH. If you smell SO2 when you add acid, then add 5g/l Sodium Sulfite. Adjust he pH.
To adjust the pH, you will need some Sodium Hydroxide solution or some Acetic Acid solution depending on the final pH.