I have not seen the consistency of China goverment's Hongkong policy. But the following is just for English learning. So please point out the best one. If none of them is best, please edit one of them to make it best.
1) The three years since Hongkong's handover have witnessed the consistency of the policy "One Country, Two Systems" laid out by Chinese Central Government.
2) The three years after the hand-over of Hongkong have witnessed a consistent implementation of the "one country, two systems" policy on the part of the Chinese central government.
3) What's been happening in HK since its handover 3 years ago is the best manifesto of central goverment's commitment to the policy of "one country, two systems".