Check first to make sure the chambers aren't already 3". It is not at all uncommon. If you do not know how to do it correctly, pay a real gunsmith to check. I would carefully check the external barrel dimensions against a 3" chambered model, and if they are close, as within thousandths, I would do it if I wanted to. You will probably get some scaredy cat advice against this, but I am a retired gunsmith and lengthened many chambers with no ill effects through the years.
Charles Daly had relabelled guns from a variety of manufacturers over the years, and the Mirokus were some of the best.