method in Form3 which starts the timer, and call it from Form1 before you call this:
However, it probably won't do what you want it to, because
is application modal: it doesn't return until the form is closed. So there is a good chance that the instance of Form3 will not update on timer Tick events until the instance of Form2 has been closed. Depends what you are trying to achieve.
You may need to use
instead, and manually ignore user inputs to Form1 until the Form2 instance has been closed.