If your A 1A2A3 is the weight itself, then A3 can be obtained by the formula =0.8-A 1-A2, and then multiplied by B 1, you can put in C 1C2C3, unless you specify to enter two (for example, A 1A2) when setting.
If your A 1A2A3 is numeric (non-proportional), then the weight needs a radix.
If you want to set two A 1A2A3 at will to get the third one, you need VBA to generate it.