AngBantamLuv
Bantam Chickens on a Pocket Prairie
There are some pretty simple designs. You have only 3, so you could go small and they'd be very excited for the change of scenery, and be safe. Maybe use an old dresser frame or table, add a couple wheels.
http://thecitychicken.com/tractors.html
http://thecitychicken.com/tractors.html