#Imports Libraries
import time
import RPi.GPIO as GPIO

#Set the GPIO pin naming mode
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

#Set pins 18, 23 and 24 to be output
GPIO.setup(18,GPIO.OUT)
GPIO.setup(23,GPIO.OUT)
GPIO.setup(24,GPIO.OUT)

#Loop forever (as true is always true)
while True:
	#Turn LEDs on
	GPIO.output(18,GPIO.HIGH)
	GPIO.output(23,GPIO.HIGH)
	GPIO.output(24,GPIO.HIGH)
	#Wait one second
	time.sleep(1)
	#Turn LEDs off
	GPIO.output(18,GPIO.LOW)
	GPIO.output(23,GPIO.LOW)
	GPIO.output(24,GPIO.LOW)
	#Wait one second
	time.sleep(1)

Create 3-blink_forever.py