NEED OF GST REGISTRATION

Dear sir

I have three proprietary firms in same PAN, in two we doing retail selling of spiritual books and in third one we doing general trading, in third firm we sell product online also that's why we take gst registration all the firm has turnover below 20 lakh,

so I wanna ask should we need to take gst registration in other firm also or we need to include the other turnover in that firm in which we take registration or what ???