A 3 week holiday - lucky you!
I take insulin all around the world. It must always be kept in hand luggage as the hold will get too cold.
I always take at least twice of everything I need including insulin, needles, lancets, test strips ... and don't forget a spare battery for your meter. If you use reusable insulin pens, take a spare one of them too.
A letter from your doctor is recommended. This is for the sharps as much as the insulin. I have been travelling with all that paraphernalia for 15 years (over 100 flights) and have only needed the letter once. However, I would much rather have the letter and not use it than to have to choose between leaving my insulin behind or not travelling.
I do not take a sharps box. Whilst I am away, I use a small drinks bottle and then decant it into the sharps box when I get home. This can go in the hold.
If you are going somewhere warm, you will need to think about keeping your insulin cool. You may have access to a fridge which is great. Alternatively, the most common solution is Frio pouches. I have had a couple of these for 15 years and use them every time I go away.
Regarding your insulin time, you have a couple of choices - continue to take it 24 hours apart or gradually move the time until it is convenient for the US. I adopt the second option: slow acting insulin usually lasts 22 to 26 hours so you can move the time a couple of hours each day without risking a major overlap. Moving it in one big step would either result in a long period of no background insulin or a long period of double dosage.
(I have read some suggesting half dosing to reduce the overlap but I have not felt comfortable with this).
My final two parts of advice are
1. make sure your diabetes is covered by your travel insurance
2. have an amazing time.