re: How are these dog breeds in regards to trainability?

The GSD and boxer will be easier to train. The GSD is likely the smartest of the three but the consequences of poor training or poor socializing are greater for that breed too. If you can commit to the development of the dog and solid training you'll be fine.

I would prefer the Boxer out of all of those b/c I like the short coats, temperament and playful, silly nature of boxers in general.