Hi Alex,
It can take about a year sometimes a little longer to heal fully and mature. If it's been 3 yrs since the old scar was formed then doing your exercises now shouldnt be a problem, however if your worried about it, i would recommend you check with the clinic your having the op with and see what they say. I had my second op last Oct, with Dr Feller and had no problems doing my exercises with my old scar which was 4 yrs old. When you have your op they will probably open your old scar anyway and close it up making a new scar.
Best of luck with your HT.
Kind Regards