Just checked my V17 installation - they are in the C:\DataCAD 17\Symbols\3D Casework subdirectory. If that doesn't exist on your box you may want to start the installation again and add them (Complete installation I think?).
Highest Regards,
Alan T. Hendry, RA
Intel i7-12700F (2.10 GHz) Desktop, 16GB RAM, 1TB SSD, Win 11 Home 64-bit, DC 23 w/ latest beta
Intel i7-10750H (2.6GHz) Laptop, 16GB RAM, 1.5TB HD, Win 11 Home 64-Bit, w/ DC 22, 23 & latest beta