Samsung galaxy s2 on three SWAP ?

I hav a samsung galaxy s2 locked the three network. phone is in very good condition/ no scratches on screen. i have no box for the phone but do hav charger and usb cable. I am looking to swap for an iphone 3gs or iphone 4. less than a year old to match condition of my phone. preferably white. Must b unlocked or on three network.