from machine import Pin
from time import sleep

greenLED = Pin(1, Pin.OUT)
yellowLED = Pin(2, Pin.OUT)
redLED = Pin(3, Pin.OUT)

while True:
    greenLED.value(1)
    yellowLED.value(0)
    redLED.value(0)
    sleep(3)

    greenLED.value(0)
    yellowLED.value(1)
    redLED.value(0)
    sleep(3)

    greenLED.value(0)
    yellowLED.value(0)
    redLED.value(1)
    sleep(3)