Create an interactive list in a Wolfram notebook that choosing its items make a change in calculations

I have 3 variables. I want to create an interactive list of items in a Wolfram notebook that if I choose each item, functions that are written by myself change the value of the variables and recalculate all the expressions. For example items are a b c and the variables are a1 a2 a3. If I choose item a, I want item a to become equal to b, etc. Then all related calculations should automatically be updated. I was wondering if anyone can help me in creating such a functionality.