Can't implement synchronization in class

Question

In the course of training, as well as with the analysis of multithreading, I decided to try my hand at thread synchronization. In this regard, I made a small test, where the creation of threads occurs through the creation of objects. And I got into a problem situation, the streams cannot be synchronized, and I don't understand what the problem is.

public class MainClass {
    static int firstInt;
    static int secondInt;
    static int thirdInt;
    public static void main(String[] args) {
        SecondClass s1 = new SecondClass("Первый");
        SecondClass s2 = new SecondClass("Второй");
        SecondClass s3 = new SecondClass("Третий");
        SecondClass s4 = new SecondClass("Четвертый");
        SecondClass s5 = new SecondClass("Пятый");

        try {
            s1.t.join();
            s2.t.join();
            s3.t.join();
            s4.t.join();
            s5.t.join();
        } catch (InterruptedException exp) {
            System.out.println("Прервано!");
        }

        System.out.println(firstInt);
        System.out.println(secondInt);
        System.out.println(thirdInt);
    }
}

class SecondClass implements Runnable {
    String name;
    Thread t;

    SecondClass(String a) {
        name = a;
        t = new Thread(this);
        t.setName(name);
        t.start();
        System.out.println("Поток с именем "+t.getName()+" создан и запущен.");
    }

    private void plusFirstInt() {
        MainClass.firstInt++;
    }

    private void plusSecondInt() {
        MainClass.secondInt++;
    }

    private void plusThirdInt() {
        MainClass.thirdInt++;
    }

    private synchronized void fori() {
        try {
            for (int i = 0; i < 10000; i++) {
                plusFirstInt();
                plusSecondInt();
                plusThirdInt();
            }
            Thread.sleep(10);
        } catch (InterruptedException exp) {
            System.out.println("Прервано!");
        }
    }

    @Override
    public void run() {
        fori();
    }
}

Tried to sync using object reference.

Object a = new Object();

private void plusFirstInt() {
    synchronized (a){
        MainClass.firstInt++;
    }
}