#include<stdio.h>
int main()
{
double a, b, c;
scanf("%3f%3f%3f", &a, &b, &c);
printf("%.3f\n", a + b + c);
printf("%.3f\n", (a + b + c) / 3.000);
return 0;
}
有问题吗？
有请说出来。

#include<stdio.h>
int main()
{
    for (int a = 1; a <= 9; a++)
        for (int b = 0; b <= 9; b++)
            for (int c = 0; c <= 9; c++)
            {
                int ABC = a * 100 + b * 10 + c;
                if (ABC < 100) continue;
                if (ABC > 999) break;
                if (ABC == a * a * a  + b * b * b + c * c * c) printf("%d", ABC);
            }
    return 0;
}

#include<stdio.h>
int main()
{
    for (int a = 1; a <= 9; a++)
        for (int b = 0; b <= 9; b++)
            for (int c = 0; c <= 9; c++)
            {
                int ABC = a * 100 + b * 10 + c;
                if (ABC < 100) continue;
                if (ABC > 999) break;
                if (ABC == a * a * a  + b * b * b + c * c * c) printf("%d", ABC);
            }
    return 0;
}

输入时不需要格式控制

double 类型是双精度的浮点型，输出输入用%lf ；float 类型是单精度浮点型 输入输出用%f。