You could start by searching the Garmin Maps forum here at GPSPower for the map version you have. The 3D file should have a link in there.
Anyone can help?
Tried searching, found few "City Navigator Singapore/Malaysia NT 2016.20 HERE" package but none of them include file "gmap3d.img"
Package only included: SQLite, SID, JCV, ASR and gmapprom.img