Pi Control Servo

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(4, GPIO.OUT)
GPIO.setup(17, GPIO.OUT)
GPIO.setup(18, GPIO.OUT)

a=GPIO.PWM(4,50)
a.start(5)

b=GPIO.PWM(17,50)
b.start(5)

c=GPIO.PWM(18,50)
c.start(5)

while True:
        #c.ChangeDutyCycle(2.5)
        a.ChangeDutyCycle(3)
        b.ChangeDutyCycle(3)
        c.ChangeDutyCycle(3)
        print'0'
        time.sleep(1)
        a.ChangeDutyCycle(7.5)
        b.ChangeDutyCycle(7.5)
        c.ChangeDutyCycle(7.5)
        print'90'
        time.sleep(1)
        a.ChangeDutyCycle(10)
        b.ChangeDutyCycle(10)
        c.ChangeDutyCycle(10)
        print'180'
        time.sleep(1)