You will need an .XANIM_EXPORT file to start with. If you do not know how to export your animation to .XANIM_EXPORT, check out the page BO3 | Export Models and Anims from Maya 8.5 and above
BO3 has a new compiled format for xanims called xanim_bin. All of your exported xanims must be converted to this format in order to be loaded by BO3 | Asset Property Editor.
In order to convert the files, you must use BO3 | export2bin.exe.
Once you have your .XANIM_BIN files, you are ready to set up the animations in BO3 | Asset Property Editor. Start by creating a new asset in your GDT of choice.
For the Asset Name, enter the name of the xanim you wish to use. I often recommend using the same name as the xanim file although it is personal preference.
For the Asset Type, choose "xanim".
Then click OK to create the xanim asset.
Once you create your xanim asset, you will see a confirmation screen with lots of options, most of which you can safely ignore.
You need to set Anim File to the XANIM_BIN file we just created, and the Model File to the XMODEL_BIN file of the xmodel which will be playing this animation.
You must select "Use Bones" if this is not a viewmodel animation, which in most cases is yes.
You must select the "Type" for the animation - in most cases this is delta. For viewmodel animations you should choose relative.
Lastly if you would like to preview your animation in BO3 | Asset Property Editor, you can scroll to the bottom of the confiruation options and set "Model" under "AssetViewer Preview Options" to the same XMODEL_BIN file you specified earlier in the confiruation. Then simply press the Play button above the 3d preview window to watch your animation.