Language: C++

#include<iostream>
using namespace std;
int main(){
	char a[100], b[100], c[100];
	int d = 1, e = 0, f = 0, g = 0;
	cin >> a;
	cin >> b;
	cin >> c;
	for (int i = 0; i < 1;){
		switch (d){
		case:1
			if (a[e] == "a")d = 1;
			else if (a[e] == "b")d = 2;
			else if (a[e] == "c")d = 3;
			else cout << "A" << endl;
			e++;
			break;
		case:2
			if (b[f] == "a")d = 1;
			else if (b[f] == "b")d = 2;
			else if (b[f] == "c")d = 3;
			else cout << "B" << endl;
			f++;
			break;
		case:3
			if (c[g] == "a")d = 1;
			else if (c[g] == "b")d = 2;
			else if (c[g] == "c")d = 3;
			else cout << "C" << endl;
			g++;
			break;
		default:
			break;
		}
	}
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:11:21: error: expected primary-expression before ':' token
   11 |                 case:1
      |                     ^
a.cc:11:23: error: expected ';' before 'if'
   11 |                 case:1
      |                       ^
      |                       ;
   12 |                         if (a[e] == "a")d = 1;
      |                         ~~
a.cc:13:25: error: 'else' without a previous 'if'
   13 |                         else if (a[e] == "b")d = 2;
      |                         ^~~~
a.cc:13:39: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   13 |                         else if (a[e] == "b")d = 2;
      |                                  ~~~~~^~~~~~
a.cc:14:39: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   14 |                         else if (a[e] == "c")d = 3;
      |                                  ~~~~~^~~~~~
a.cc:18:21: error: expected primary-expression before ':' token
   18 |                 case:2
      |                     ^
a.cc:18:23: error: expected ';' before 'if'
   18 |                 case:2
      |                       ^
      |                       ;
   19 |                         if (b[f] == "a")d = 1;
      |                         ~~
a.cc:20:25: error: 'else' without a previous 'if'
   20 |                         else if (b[f] == "b")d = 2;
      |                         ^~~~
a.cc:20:39: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   20 |                         else if (b[f] == "b")d = 2;
      |                                  ~~~~~^~~~~~
a.cc:21:39: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   21 |                         else if (b[f] == "c")d = 3;
      |                                  ~~~~~^~~~~~
a.cc:25:21: error: expected primary-expression before ':' token
   25 |                 case:3
      |                     ^
a.cc:25:23: error: expected ';' before 'if'
   25 |                 case:3
      |                       ^
      |                       ;
   26 |                         if (c[g] == "a")d = 1;
      |                         ~~
a.cc:27:25: error: 'else' without a previous 'if'
   27 |                         else if (c[g] == "b")d = 2;
      |                         ^~~~
a.cc:27:39: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   27 |                         else if (c[g] == "b")d = 2;
      |                                  ~~~~~^~~~~~
a.cc:28:39: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   28 |                         else if (c[g] == "c")d = 3;
      |                                  ~~~~~^~~~~~