Language: C++

#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define eps 1e-9
#define maxp 99940000
#define mod 1000000007
#define pi 3.141592653589793238
#define sin(s) scanf("%lld",&s);
#define sout(s) printf("%lld\n",s);
#define in(s) cin>>s
#define in2(a,b) cin>>a>>b
#define in3(a,b,c) cin>>a>>b>>c
#define vll vector<ll>
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define endl '\n'
#define sp ' '
#define gear_change ios_base::sync_with_stdio(false),cin.tie(0)
#define nfl(i,n) for(i=0;i<n;i++)
#define sfl(i,s,e) for(i=s;i<e;i++)
#define ufl(i,n) for(i=n-1;i>=0;i--)
ll power(ll x,ll y){
    ll ret=1;
    while(y>0){
        if(y&1)
            ret=(ret*x);
        y/=2;
        x=(x*x);
    }
    return ret;
}
string rot(string s){
	string ret="";
	ll i;
	for(i=1;s[i];i++){
		ret+=s[i];
	}
	ret+=s[0];
	return ret;
}
ll power(ll x,ll y,ll p)
{
	ll ret=1;
	while(y>0)
	{
		if(y&1)
			ret=(ret*x)%p;
		y/=2;
		x=(x*x)%p;
	}
	return ret%p;
}
void solve(){
	ll n,i;
	string s;
	cin>>s;
	cout<<s.substr(0,3)<<endl;
}	
int main(){
	gear_change;#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define eps 1e-9
#define maxp 99940000
#define mod 1000000007
#define pi 3.141592653589793238
#define sin(s) scanf("%lld",&s);
#define sout(s) printf("%lld\n",s);
#define in(s) cin>>s
#define in2(a,b) cin>>a>>b
#define in3(a,b,c) cin>>a>>b>>c
#define vll vector<ll>
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define endl '\n'
#define sp ' '
#define gear_change ios_base::sync_with_stdio(false),cin.tie(0)
#define nfl(i,n) for(i=0;i<n;i++)
#define sfl(i,s,e) for(i=s;i<e;i++)
#define ufl(i,n) for(i=n-1;i>=0;i--)
ll power(ll x,ll y){
    ll ret=1;
    while(y>0){
        if(y&1)
            ret=(ret*x);
        y/=2;
        x=(x*x);
    }
    return ret;
}
string rot(string s){
	string ret="";
	ll i;
	for(i=1;s[i];i++){
		ret+=s[i];
	}
	ret+=s[0];
	return ret;
}
ll power(ll x,ll y,ll p)
{
	ll ret=1;
	while(y>0)
	{
		if(y&1)
			ret=(ret*x)%p;
		y/=2;
		x=(x*x)%p;
	}
	return ret%p;
}
void solve(){
	ll n,i;
	string s;
	cin>>s;
	cout<<s.substr(0,3)<<endl;
}	
int main(){
	gear_change;
	ll test=1;
	// in(test);
	while(test--){
		solve();
	}
}	

	ll test=1;
	// in(test);
	while(test--){
		solve();
	}
}	


Compiler output:
a.cc:63:21: error: stray '#' in program
   63 |         gear_change;#include<bits/stdc++.h>
      |                     ^
a.cc: In function 'int main()':
a.cc:63:22: error: 'include' was not declared in this scope
   63 |         gear_change;#include<bits/stdc++.h>
      |                      ^~~~~~~
a.cc:63:30: error: 'bits' was not declared in this scope
   63 |         gear_change;#include<bits/stdc++.h>
      |                              ^~~~
a.cc:63:35: error: 'stdc' was not declared in this scope; did you mean 'std'?
   63 |         gear_change;#include<bits/stdc++.h>
      |                                   ^~~~
      |                                   std
a.cc:64:1: error: expected primary-expression before 'using'
   64 | using namespace std;
      | ^~~~~
a.cc:87:20: error: a function-definition is not allowed here before '{' token
   87 | ll power(ll x,ll y){
      |                    ^
a.cc:97:21: error: a function-definition is not allowed here before '{' token
   97 | string rot(string s){
      |                     ^
a.cc:107:1: error: a function-definition is not allowed here before '{' token
  107 | {
      | ^
a.cc:118:13: error: a function-definition is not allowed here before '{' token
  118 | void solve(){
      |             ^
a.cc:124:9: warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse]
  124 | int main(){
      |         ^~
a.cc:124:9: note: remove parentheses to default-initialize a variable
  124 | int main(){
      |         ^~
      |         --
a.cc:124:9: note: or replace parentheses with braces to value-initialize a variable
a.cc:124:11: error: a function-definition is not allowed here before '{' token
  124 | int main(){
      |           ^