I want to make BO3 maps but I think what would work best for me is learning with another program because the call of duty resources are spread thin. I'd love to start with an engine similar to radiant that has a lot of resources to learn. One that encourages coding and map design. Thank you for any input! 

You might want to have a look at Quake 3 as the level editor used to make maps for it is another version of Radiant, or you can have a look at Source Engine, which is, just like Call of Duty, a fork of the Quake Engine. Both of them have very different ways of creating content, but if you want to get good at BO3 mapping, I suggest you stick to that, and that alone.