from machine import Pin, Timer

red=Pin(0,Pin.OUT)
green=Pin(2,Pin.OUT)
yellow=Pin(6,Pin.OUT)

flag=0;

def cycle_leds(timer):
    global flag
    red.off()
    green.off()
    yellow.off()
    if flag == 0:
        red.on()
    elif flag == 1:
        green.on()
    elif flag == 2:
        yellow.on()

    flag = (flag + 1) % 3 


timer = Timer(period=1000, mode=Timer.PERIODIC, callback=cycle_leds)