我打的复杂了，题目圈定了给的是3位数，不过代码可以应对更大的数，不信的可以试（小于int范围）

#pragma GCC optimize(2)
#include <iostream>
#include <cstring>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
    ios::sync_with_stdio(0);
    cout.tie(0); cin.tie(0);
    //int n; cin >> n;
    vector<char> v;
    
    char c;
    while (cin >> c) v.push_back(c);
    
    int res = 0;
    for ( int i = 0; i < v.size(); i ++ ) res += v[i] - '0';
    
    cout << res;
    return 0;
}

#include <iostream>
using namespace std;

int main()
{
int e,f,g,y;
cin>>e;
f=e%10;
g=e/10%10;
y=e/100;

cout<<f+g+y;

return 0;

}

#include <iostream>
using namespace std;
int main()
{
int x,a,b,c;

cin>>x;
a=x%10;
b=x/10%10;
c=x/100;
cout<<a+b+c;

return 0;
}