If I understand you correctly, you want to have a parent table that is related to a child table, one to many.
Then, the child table, related to a grandchild table, one to many.
In other words, you have nested relationships.
Based on this article, I think that the widgets should support your workflow:
See 'Related Data in ArcGIS' Blog post by Michael Kelly on geonet:
o https://community.esri.com/people/MKellyesri-ireland-ie-esridist/blog/2016/11/18/related-data-in-arc...
Try testing it in the wab widget.
The related table should be visible under the 'Actions' heading> table icon (view related layers and tables)
Click the table icon to see the first related table.
Click the table icon again to see the second related table, if possible.
My personal experience is to have a parent feature class that is related to a child table, one to many.
Then creating attachments on the child table (which to me, is very similar to having a related grandchild to the child)
This workflow works in wab smart editor widget.
The exception is that I cannot drill down to see the attachment table in the widget, but that may be different for you since, yours is a regular table, not a table like the attachment table, which was auto-created by the geodatabase.