Hi thanks for your question.
Your periods are delayed by 3 weeks and your 3 pregnancy
tests are negative.There is nothing to worry tension is one of common causes of delayed periods and secondly lactation
also delays periods.Rule out pregnancy by ultra sound
abdomen and then you can you take OCP
,in consultation with your gynaecologist,which will give you protection against pregnancy and periods will also become regular.
Hope this answers your question.