Problem p02645 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define _CRT_SECURE_NO_DEPRECATE
#define ll long long
#define nl '\n' // NL - sign of new line

#define upc(str, c) str[c] = toupper(str[c]);    // make single char uppercase
#define lowerc(str, c) str[c] = tolower(str[c]); // make single char lowercase

#define ups(str)                                                               \
  transform(str.begin(), str.end(), str.begin(),                               \
            ::toupper); // make entire string uppercase
#define lowers(str)                                                            \
  transform(str.begin(), str.end(), str.begin(),                               \
            ::tolower); // make entire string lowercase

#define ps(x, y)                                                               \
  fixed << setprecision(y) << x // usage: t=1.1234; cout << ps(t,4); // it means
                                // it will print 4 decimal places.

#define vi vector<int>    // create vector of int numbers
#define vs vector<string> // create vector of int numbers
#define eb emplace_back
#define rev(v) reverse(v.begin(), v.end()) // reverse the vector

#define mii std::map<int, int>
#define multimii std::multimap<int, int>
#define mss map<string, string>

#define w(t)                                                                   \
  unsigned int t;                                                              \
  cin >> t;                                                                    \
  while (t--) //  usage: create quickly number of test
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

void c_p_c() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
}
//==================================6

//==================================
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  c_p_c();
  //--------------------------------

  char s[3];
  cin >> s;

  cout << s[0] << s[1] << s[2];

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define _CRT_SECURE_NO_DEPRECATE
#define ll long long
#define nl '\n' // NL - sign of new line

#define upc(str, c) str[c] = toupper(str[c]);    // make single char uppercase
#define lowerc(str, c) str[c] = tolower(str[c]); // make single char lowercase

#define ups(str)                                                               \
  transform(str.begin(), str.end(), str.begin(),                               \
            ::toupper); // make entire string uppercase
#define lowers(str)                                                            \
  transform(str.begin(), str.end(), str.begin(),                               \
            ::tolower); // make entire string lowercase

#define ps(x, y)                                                               \
  fixed << setprecision(y) << x // usage: t=1.1234; cout << ps(t,4); // it means
                                // it will print 4 decimal places.

#define vi vector<int>    // create vector of int numbers
#define vs vector<string> // create vector of int numbers
#define eb emplace_back
#define rev(v) reverse(v.begin(), v.end()) // reverse the vector

#define mii std::map<int, int>
#define multimii std::multimap<int, int>
#define mss map<string, string>

#define w(t)                                                                   \
  unsigned int t;                                                              \
  cin >> t;                                                                    \
  while (t--) //  usage: create quickly number of test
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

void c_p_c() {
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
}
//==================================6

//==================================
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  c_p_c();
  //--------------------------------

  string s;
  cin >> s;

  cout << s[0] << s[1] << s[2];

  return 0;
}