I would not let them stack. 1 Focus item for control, 1 for power, no more, no less.
They are different tools for the same job. Just because you've got 3 different sized hammers in your hand doesn't mean you'll get the nail into the wall any better than just taking the best hammer for the job. If the overlap is too frequently, I would see that I switch them out. So in your case, just turn it into +3 summer fire permanently, and everything is fine.