Go to cbec website, Download CGST rules, it has format of GSTR-3B.
You can check the format in the article GSTR-3B format and filing the return
You can also login to GST portal and check under Return actual format.
You need not to upload invoice details for filing form GSTR-3B, however you have to provide details of each B2B invoice in GSTR-1.
You can download GSTR-1 offline utility from GST portal, they have provide CSV files fill it, import in utility and file the return.
For B2C customers you need to provide only consolidated details.