Final answer:
The method declaration for a public class method named calculateAverage() with double parameters and return value.
Step-by-step explanation:
The method declaration for a public class method named calculateAverage() that has double parameters firstNum, secondNum, thirdNum and returns a double value can be written as:
public static double calculateAverage(double firstNum, double secondNum, double thirdNum) {
//code to calculate the average of the three numbers goes here
return average;
}