where did i make the problem in declaring the function average in this code c++

#include <iostream>

using namespace std;

double Average(inta, intb, intc);


int main()
{

    int num1, num2,num3,z;

    cout<<"enter three numbers";

    cin>>num1>>num2>>num3;

    z=Average(num1,num2,num3);

    cout<<"Average="<<z;

    return 0;
}