This worked for me and all my friends. If your beta code is not working here is how to fix it.
1) log on to xbox on your computer, make sure your on your account and go to the xbox live tab and click redeem

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2) type in your 25 digit code it wont work hit close
3) scroll all the way to the bottom to where it says united states and click that and switch it to Brazil

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4) it will be kind of hard to read but it will keep you on the redeem page, just click the big green redeem picture, it will be in Brazil of course, and enter your code and click enter. it should say call of duty black ops 3 beta then some stuff in Brazil. click the green button at the bottom saying confirm in Brazil. then it will say success
5) go on to your xbox console and go to ready to install and click call of duty black ops 3 and there you go! it should start working.

You're welcome, if it doesn't work im sorry. but this did work for me and all of my friends