按题意直接模拟即可。

AC code：

#include <bits/stdc++.h>
using namespace std;

int main() {
    long long a, b, c;//不开long long见祖宗
    cin >> a >> b >> c;
    if (a == b || b == c || a == c) {
        return 0;
    } else {
        if (a > b && a > c) {
            cout << "A" << endl;
        } else if (b > a && b > c) {
            cout << "B" << endl;
        } else {
            cout << "C" << endl;
        }
    }
    return 0;
}