one of my freind filed 3b without adding sales and purchase details where purchase is more than sales in the month of June 2019 but he added bill in gstr1 how can I add those bill and claim itc.
If he added sales invoice in GSTR 1 but not reported in GSTR 3B than you can report that amount in current GSTR3B in output tax Column along with current month's sale and Pay tax accordingly... If you forgot to take ITC or ITC not taken relating to invoice of F.Y 2019-20 than you can take that ITC in current month GSTR 3B..
You have / can do all this on or before 20th October, 2020 (i.e due date of filing GSTR 3B of September, 2020)