My name is***** am sorry to hear that you are having this issue. I will be glad to assist you with this.
Try these steps please.
This sounds like your 3rd Kindle is set as the default Deliver To device.
Go to your Amazon account on your computer and go to Manage content and devices. Then click on the book and there will be a deliver to box, click on that to get the drop down menu that has all of the devices in the list and choose the new Kindle. If the new Kindle is not listed, make sure it is registered to the same Amazon account. You can check that on the kindle menu from the home screen, then settings,registration.
Did that work for you?