You Can Cancel Order After You Play the Beta
Just because you pre-order for the Call of Duty: Black Ops 3 beta doesn't mean you need to buy the game. If you don't like playing the new Call of Duty 2015 release during the beta and you want to cancel you can still do that. Keep in mind that the beta is a test and there will be changes to the game so don't judge weapon balances completely or the online gaming experience completely because both will likely change ahead of a final release.
If you think you might go this route, you should buy Call of Duty: Black Ops 3 on Amazon where it is simpler to cancel a pre-order than at many other stores.