input: Write a python function to draw circles with different radii using the turtle module.
  assert draw_circles([(0, 0, 20), (0, 0, 30), (0, 0, 40)]) is None
  assert draw_circles([(50, 50, 10), (100, 100, 20), (150, 150, 30)]) is None
  assert draw_circles([(0, 0, 50), (100, 100, 50), (200, 200, 50)]) is None
output: import turtle
  
  def draw_circles(circles):
      screen = turtle.Screen()
      pen = turtle.Turtle()
      
      for circle in circles:
          x, y, radius = circle
          pen.penup()
          pen.goto(x, y - radius)
          pen.pendown()
          pen.circle(radius)
      
      screen.mainloop()