Answer:
The function is as follows:
double testAverage(double n1, double n2, double n3, double n4){
  double small = findLowest(n1,n2,n3,n4);
  double total = n1 + n2 + n3+ n4 - small;
  double ave = total/3;
  return ave;
}
Explanation:
This defines the function
double testAverage(double n1, double n2, double n3, double n4){
This calls the findLowesr function and save the smallest in variable small
  double small = findLowest(n1,n2,n3,n4);
This calculates the sum of the largest 3
  double total = n1 + n2 + n3+ n4 - small;
This calculates the average of the largest 3
  double ave = total/3;
This returns the calculated average
  return ave;
}
Note that: I assume that function findLowest exists (according to the question).