I smoke 3 cigarettes per day after 20 days of my hair transplant it is very dangerous for newly graft?

It is not dangerous to smoke cigarettes but most doctors would encourage you to stop smoking in general.

Assuming you're starting after 20 days, no, you aren't doing anything to specifically harm your grafts. However, smoking regularly can negatively impact your hair growth/loss overall, both the transplanted and natural hairs, as well as have negative impacts upon a variety of other health factors.

I am sure that I don't need to tell you that smoking is harmful to your health, but 3 cigarettes per day will not have any adverse effect on your grafts. It definitely won't help, but it wont hurt too bad either. See link below for more information. 

Smoking after 3 weeks probably will do no harm to your transplant but more harm to your overall health