Using GA for EXE programs

I am very new on Genetic algorithms; I am going to use GA for an optimization problem. I have a text file that has 3 input parameters and one output parameter. Each time I can run an exe program to generate this text file. I am trying to find the best set of input parameters so that I have the minimum value of the output parameter. How can I do this in MATLAB? I do not know what would be the fitness function, and how should I assign it? Could you please help me with this?

Thank you