i am makeing a traffic light with python turtle and at the end of the sequance i want all the lights to blink yellow : this is my code all i want is for three turtles to move/blink at the same time 
like a broken traffic light.
i cant figure out how to do it.
if you want to see all of my code let me now by an answer


   

    #flash
    speed(20)
    #pens
    pen1 = Turtle()
    pen2 = Turtle()
    pen3 = Turtle()
    #loop
    def loop():
    
    while True:
        pen1.penup()
        pen1.color('yellow')
        pen1.goto(0,90)
        pen1.begin_fill()
        pen1.pendown()
        pen1.circle(40)
        pen1.penup()
        pen1.end_fill()
        pen1.color('black')
        time.sleep(1)
        pen1.begin_fill()
        pen1.pendown()
        pen1.circle(40)
        pen1.penup()
        pen1.end_fill()
        time.sleep(1)
    while True:
        pen2.penup()
        pen2.color('yellow')
        pen2.goto(0,-140)
        pen2.begin_fill()
        pen2.pendown()
        pen2.circle(40)
        pen2.penup()
        pen2.end_fill()
        pen2.color('black')
        time.sleep(1)
        pen2.begin_fill()
        pen2.pendown()
        pen2.circle(40)
        pen2.penup()
        pen2.end_fill()
        time.sleep(1)



    return loop()


    

         