Assuming "A", "B", and "C" are numeric outputs from a calculation, you can use a formula node.
Here is a quick example based off of your (somewhat confusing) notation. It may need altered slightly, but it could be a starting point (assuming I understood you correctly and you are only dealing with simple, numeric data types as described in your example).
Otherwise you could just do this using regular LabVIEW Comparison Functions and Dataflow.