Thanks for the query to ATD.
PLAN-B is an emergency contraceptive pill and is not be used at all post delivery as you are at present under lactational phase and hence ovulation shall not occur keeping you safe from pregnancy naturally as its just been 3 weeks now. If at all contraception is a concern then you may opt for 3 monthly Inj. Depot Provera, which shall help avoid pregnancy as long as you want and its safe and advisable especially in post partum period.
I hope I have answered your query in detail,
Wishing you good health,
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