Survey123 Connect can create forms from existing feature services. Unfortunately, currently, the functionality can't be utilized for Feature Services from 3D Multipatch Data. I assume it's because one of the field from multipatch is identified as an unrecognizeable attribute for S123 Connect smartforms.
You are able to find your existing feature service from AGOL/Enterprise org, but it will fail when convert to s123 XLSForm.
This functionality will offers rich opportunities of utilizing s123 forms with 3D GIS implementation