class MyClass {

  private ExternalClass1 ex1;
  private ExternalClass2 ex2;
  private ExternalClass3 ex3

public String doSomething(String arg1){
 val1=ex1.invoke(arg1);
 val2=ex2.call(val1);
 result=ex3.doit(val2);
 return result;
}

}

@Test
void doSometing(){
   ExternalClass1 ex1=mock(ExternalClass1);
   ExternalClass2 ex2=mock(ExternalClass2);
   ExternalClass3 ex3=mock(ExternalClass2);

when(ex1.invoke(arg1)).thenReturn(val1);
when(ex2.call(val1)).thenReturn(val2);
when(ex3.doit(val2)).thenReturn(result);

Myclass myClass=new MyClass(ex1,ex2,ex3);

assertEquals(result,myClass.doSomething(arg1))
}