According to Bethesda’s blog and Fallout 4’s product page on Xbox.com, if you pre-order your copy of Fallout 4 for your Xbox One you’ll receive a free copy of Fallout 3.
After pre-ordering your copy you will be entitled to a copy of Fallout 3. You’ll receive your code in you Xbox message center after about 7-10 days. After getting your code you can either redeem it on Xbox.com or your Xbox 360. Fallout 3 will also be backwards compatible on your Xbox One once it’s made available.
The pre-order needs to be made through the Xbox main site and can be done so here. Fallout 4 will be released on November 10 on PlayStation 4, Xbox One, and PC.