ALX23z thank you for the explanation I adjusted my code and now it works

#include <iostream>
#include <mutex>
#include <condition_variable>
#include <thread>
#include <chrono>

using namespace std;

class Monitor
{
private:
    string line = "";
    mutex mtx;
    condition_variable cv;
    int vowel_count = 0;
    int max_count = 0;

public:
    void AddA(char symbol)
    {
        unique_lock<mutex> lock(mtx);
        cv.wait(lock, [this] { return vowel_count < 3; });

        if (max_count >= 15)
        {
            return;
        }

        line += symbol;
        vowel_count++;
        max_count++;
        cv.notify_all(); 
    }

    void AddBC(char symbol)
    {
        unique_lock<mutex> lock(mtx);
        cv.wait(lock, [this] { return vowel_count >= 3; });

        if (max_count >= 15)
        {
            return;
        }

        line += symbol;
        max_count++;
        vowel_count = 0;
        cv.notify_all();
    }

    string getLine()
    {
        unique_lock<mutex> lock(mtx);
        return line;
    }

    int getVowelCount()
    {
        unique_lock<mutex> lock(mtx);
        return vowel_count;
    }

    int getMaxCount()
    {
        unique_lock<mutex> lock(mtx);
        return max_count;
    }
};

void First(Monitor& monitor)
{
    while (true)
    {
        if (monitor.getMaxCount() >= 15)
        {
            break;
        }

        monitor.AddA('A');

        std::this_thread::sleep_for(std::chrono::milliseconds(100));
    }
}

void Second(Monitor& monitor)
{
    while (true)
    {
        if (monitor.getMaxCount() >= 15)
        {
            break;
        }
        monitor.AddBC('B');

        std::this_thread::sleep_for(std::chrono::milliseconds(100));
    }
}

void Third(Monitor& monitor)
{
    while (true)
    {
        if (monitor.getMaxCount() >= 15)
        {
            break;
        }

        monitor.AddBC('C');

        std::this_thread::sleep_for(std::chrono::milliseconds(100));
    }
}

void Print(Monitor& monitor)
{
    while (true)
    {
        cout << monitor.getLine() << endl;

        if (monitor.getMaxCount() >= 15)
        {
            break;
        }

        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    }
}

int main()
{
    Monitor monitor;

    thread t1(First, ref(monitor));
    thread t2(Second, ref(monitor));
    thread t3(Third, ref(monitor));
    thread t4(Print, ref(monitor));

    t1.join();
    t2.join();
    t3.join();
    t4.join();

    cout << "Done" << endl;
    cout << monitor.getLine() << endl;

    return 0;
}