Question

Let's suppose that I have A,B,C, each one with a percentage like A=0.5 B=0.2 C=0.3
And now I want to update those values with an input like: A'=0.4 B'=0.3 C'=0.3
So the logic would be that A should go down a little bit, B goes up a little bit, and C mantains the value. How can I achieve this? With the merge, the SUM of A B and C shall remain 1 (or a 100%).