If you are using the NYC Geodatabase with QGIS 3.2 (and possibly 3.0) and are not able to view certain layers (i.e. you drag them into the map view and nothing appears), this is due to some bug with how QGIS reads Spatialite layers that have spatial indexes. The following layers are affected: a_pumas2010, a_tracts, and a_zctas. To get them to display, you can disable the spatial index. You can do this is the Spatialite GUI or the QGIS DB Manager by running this command in the SQL window:
SELECT DisableSpatialIndex('LAYER', 'geometry');
Where LAYER is the name of the layer in quotes, i.e. 'a_tracts'. Run this command on each of the three layers. Then refresh the database or remove the connection and re-establish it, and try adding the layers to the view. You should be back in business.
Alternatively, you could go back to using QGIS 2.18, which is still the long term release and is inherently more stable and hassle free.