from machine import Pin,Timer
import utime

def fn(timer):
    led1=Pin(2,Pin.OUT)
    led1.on()
    utime.sleep(1)
    led1.off()
    led2=Pin(3,Pin.OUT)
    led2.on()
    utime.sleep(1)
    led2.off()
    led3=Pin(4,Pin.OUT)
    led3.on()
    utime.sleep(1)
    led3.off()
    
timer=Timer(freq=1/3,mode=Timer.PERIODIC,callback=fn)