How to write a search to return a value from one of three columns in a CSV lookup based on a field?

I am trying to write a lookup that will pull a value out from one of three different columns. for example Col_A, Val_A, Val_B, Val_C I want to look up only one of the values depending on a field. So if lookup_field = Val_B , I want to use Col_A and pull out value from Val_B column.