Randomized selection of vector elements

In MyClass class, the constructor calls generate to set the value of another class member, which works fine. The issue occurs when I try to generate 9(3 X 3) strings from MyClass::generate() when called from another class, using:

OtherClass::OtherClass

  std::vector<MyClass> getString;
  getString.reserve(3);

  for(int j = 0; j < 3; j++){
      for(int i = 0; i < 3; i++){
          MyClass myclass;
          getString.push_back(myclass);
       }
       mapContainer[keyFromVector.at(j)] = getString;
       getString.clear();
   }

MyClass::generate()

std::vector<std::string> listOfString;

std::string MyClass::generate(){

    listOfString = { "string1", "string2",....., "string20" };

    std::random_device random;
    std::mt19937 rand_string(random());
    std::shuffle(listOfString.begin(), listOfString.end(), rand_string);
    std::string getString = listOfString.at(1);

    listOfString.erase(listOfString.begin() + 1);  

    return getString;
}

What I want as output is:

Key1

String1, String5, string7

Key2

String2, string13, string4

Key3

string10, string17, string11

Right now, I get repeated string values from