CBIC had announced quarterly system of GSTR 3B by way of amendment to GST rules.
Later, a notification was issued wherein various features of this scheme were discussed, as to who will be eligible for this system of return filing, two methods were announced for monthly payment of tax etc.
Although the above option has been made available but the process for opt in would start from 05.12.2020 as can be seen in below image:
Also, even though this facility might give you benefit of filing quarterly GSTR 3B but do remember you have to still make all calculation and pay tax using the challan or if you are using fixed sum method then you just have to pay challan.
Thus, all the GST experts should be wise and evaluate both scenarios before opting this for your client, as it might save some cash flow but this can make things very confusing.