looks good to me - I'm not a boxer and have never trained like one, but I think keeping your strength training basic while performing or practicing your skill (boxing) is in line with what's recommended. You may even want to try the twice a week template for strength since you're practicing boxing 2-3 times per week. I'd keep 1 assistance and 1-2 accessory lifts, as you're already doing; just be sure they address a weakness, or build on your particular strength. I'd do bodyweight assistance/accessories (like dips, chins, pushups, abs) as they are very complimentary to boxing.
For those more experienced in boxing and strength training, I'd be curious what they'd recommend by means of where in the training schedule to place strength training and boxing. You don't want to be slow and tired when boxing, so I'd assume you'd want to do boxing before strength training, like this:
I could be way off since like I said, I've never trained like a boxer - but I'm simply trying to apply some basic principles and common sense with regards to strength training while practicing/training a skill/sport.