>>626
これでいいかな？

#include<stdio.h>

double heikin(double, double, double);

int main(){
double x,y,z;

printf("3つの数を入力\n");
printf("x = ");
scanf("%lf",&x);
printf("y = ");
scanf("%lf",&y);
printf("z = ");
scanf("%lf",&z);

printf("3つの数の平均は%fです\n",heikin(x,y,z));
}

double heikin(double x, double y, double z){
return (x+y+z)/3;
}