This discussion clarifies how to report Reverse Charge Mechanism (RCM) supplies in GSTR 3B. For tax payable under RCM, it should be shown in Table 3.1(d). Input tax credit for RCM tax is reported in Table 4(A)(3). A self-invoice is required for RCM payments. While suppliers of services under RCM (like music composers) may not pay tax directly, it's important to understand how this affects GSTR-1 and GSTR 3B reporting to avoid turnover mismatches.
In my opinion , ideally RCM outward supply is to be reported in GSTR 3B in 3.1.a . However portal will throw error is tax col is blank and hence report in 3.1.c