3SUM

#include <bits/stdc++.h>

using namespace std;
#define int long long
#define vout(x) for(int i=0;i<(long long)x.size();i++) printf("%lld ",x[i]);

// g++ <filename>.cpp -g -Wall -Wextra -DDEBUG -o <executable>

// copied from: https://codeforces.com/blog/entry/79024
// === Debug macro starts here ===

int recur_depth = 0;
#ifdef DEBUG
#define dbg(x) {++recur_depth; auto x_=x; --recur_depth; cerr<<string(recur_depth, '\t')<<"\e[91m"<<__func__<<":"<<__LINE__<<"\t"<<#x<<" = "<<x_<<"\e[39m"<<endl;}
#else
#define dbg(x)
#endif
template<typename Ostream, typename Cont>
typename enable_if<is_same<Ostream,ostream>::value, Ostream&>::type operator<<(Ostream& os,  const Cont& v){
	os<<"[";
	for(auto& x:v){os<<x<<", ";}
	return os<<"]";
}
template<typename Ostream, typename ...Ts>
Ostream& operator<<(Ostream& os,  const pair<Ts...>& p){
	return os<<"{"<<p.first<<", "<<p.second<<"}";
}

// === Debug macro ends here ===

signed main() {
    
    int n, x;
    cin >> n >> x;

    vector<pair<int,int>> vec(n);
    
    for (int i = 0; i < n; i++) {
        int a;
        cin >> a;
        vec[i] = make_pair(a, i+1);
    }

    sort(vec.begin(), vec.end());

    vector<int> res;

    for (int i = 0; i < n; i++) {
        
        int l = i+1;
        int r = n-1;

        while (l < r) {
            if (vec[i].first + vec[l].first + vec[r].first == x) {
                // cout << vec[i].second << " " << vec[l].second << " " << vec[r].second << endl;
                res.push_back(vec[i].second);
                res.push_back(vec[l].second);
                res.push_back(vec[r].second);

                sort(res.begin(), res.end());
                vout(res);
                cout << "\n";

                return 0;
            } else if (vec[i].first + vec[l].first + vec[r].first < x) {
                l++;
            } else {
                r--;
            }
        }
    }

    cout << "IMPOSSIBLE" << "\n";

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED