convertir 2 cadenas de minúsculas y otra de mayúsculas y las compare y diga si son iguales

void stringCompare(string a, string b, string c) {
    string a1 = tolower(a);
    string b1 = tolower(b);
    if (a1 == b1) {
        cout << "The lower case strings are equal." << endl;
    } else if (a1 == c) {
        cout << "The first string and the uppercase string are equal." << endl;
    } else if (b1 == c) {
        cout << "The second string and the uppercase string are equal." << endl;
    } else {
        cout << "The three strings are not equal." << endl;
    }
}

stringCompare(string1, string2, string3);