You have to create your personal 3D libraries exactly the same way as you create symbol and footprint libraries.
Read this FAQ.
After you have created a/some 3D libraries for ALL modified and imported 3D models, you then open the properties on any footprint to find this:
Select “3D models”… see red arrow.
Click the grey banner… green arrow, to change or select models from libraries.
Steer to, add and configure paths to any created 3D libraries with the boxes pointed to with the magenta arrows. This section works exactly the same way as the personal symbol and footprint libraries referred to in the FAQ link.