Noun form of "affected", meaning things that are affected by X

Solution 1:

I think that subject would work in such a sentence:

A, B and C are important because they are the common subjects to process X.

Solution 2:

Here are a few proposals to help you express your ideas in your thesis.

1. A, B and C are important because they are the common reformates of process X. (Borrowed from chemistry.)

2. A, B and C are important because they are simultaneously affected by process X.

3. A, B and C are important because they are the common processes affected by X. (I don't know if A, B and C are processes, but whatever they are, that's the word to put after "common.")

4. A, B and C are important because they are the common products of process X.

5. Modelling your situation as a directed graph could be helpful. Draw a directed graph with arrows going from X to A, X to B, and X to C. Explain that an arrow pointing from vertex P to vertex Q will mean that process P affects process Q. Then you can say that A, B and C are important because they are the terminal vertices of X.