Your problem is that once your function prototype for 'calcAverage' is different from its definition. Also, you're just calling that function ( which returns a value ) and not assigning it to anything.
Change the function call of calcAverage to: average = calcAverage(score1, score2, score3);
Then it will work.
You should consider changing how you assign your score values though, the function 'getTestScores' isn't really using functions like you're supposed to. Arguments passed to a function usually contain a value that will be usefull to that function. You might be better off making 'getTestScores' more general and calling it 3 times within a loop from main, or just get the values from within main and ridding yourself of the function. Just something to think about.