Hi Andrea. I had a double mastectomy and reconstruction in April this year. However I started with a wide local excision and then a re-excision a week later for margins. No axilliary clearance. I had good range of movement (though not normal) immediately. You have to do the exercises the physios give you every day, they help a lot. If there are no problems you'll probably find three weeks is a good amount of time to recover reasonably well. Physically anyway. Emotionally it can take a lot longer. Some people are good after two, some after four. We're all different.
You'll need to prep your house so you don't have to reach too high to get something! You might need a hand washing yourself for the first few days.
Something you should mentally prepare yourself for is the possibility that the size of your tumours may be larger than estimated from the mammogram. This can change your treatment plan.
My sister died from BC 17 months before I was diagnosed. I understand your terror. We've all felt that cold hand grip our guts. I can promise you that it gets better. Once treatment is underway you adjust, and move forward. We're with you here. Stay in touch. Big hug, K xox