Tom a simple, suitable means to most of your needs for ascertaining the combined CE of your sail plan would be to scale out the boat with the mast, sails etc in their relative position, geometrically draw the C of E of each sail, scale the point relative to the combination of all the sails to find the point at which the Cof E of all of them acts then project that down through the hull. For the purpose that it appears that you need it should be accurate enough to assume that all three sails are flat thereby not needing to go into more complicated calculations allowing for the differences in their shape and "drive" positions etc. You can do this with only one sail or with two or with all three, then you can see a good comparison with how the centre of effort of the different combionations will effect the balance.