You have to implement your own synchronization mechanism to do so.
A very simple implementation can be as follows. Note that you can write a lot more elegant way of doing it.

public class A {
  private int count = 0;

  public void method() {
     if(getActiveThread() < 3) {
        methodHelper();
     }
  }

  private synchronized void methodHelper() {
     i++;
     // method implementation here.
     i--;
   }

  private synchronized int getActiveThread() {
    return i;
  }
}