I cannot answer your question because I need more information to properly calculate common vent pipe (the chimney flue) minimum and max diameter.
There are some important definitions at the beginning of the document (first few pages) and than your example would probably be on page 20.
Example #3 if all of your appliances have draft hoods
Example #4 if they are mixed; draft hood (water heaters) + induced draft motor (maybe your furnace)
If one of the above is your setup (they apply to more than two appliances as well), you’d scroll to page #13 and read the values from “Common Vent Capacity” table. You need to know the total vertical height of the chimney (measuring from the top of the appliance) to use Gama tables.
Let me know if you have any problems with calculations.