Hi Sami Ji,
Every registred person has to file GSTR1 (if annual turnover exceeds Rs. 1.5 crores) and GSTR3B on monthly basis.
GSTR2A is based on GSTR1 filled by the party from whom you made purchase and it is auto populated you have to do nothing.
for further query or issue feel free to contact at +91-8587078217 (CA Gaurav Gupta)