When Creating a 3d Postcard the Image Darkens

When you switch to 3D, how the 3D scene looks now depends on what is lighting the 3D scene. You want to add at least one light of some type to shade the 3D scene the way you want it. How light or dark any of the 3D objects is depends on both the distance/intensity of the lights, and the angle of the current camera, and of course some types of light might cause light falloff (less light toward one or more edges).

This principle is the same in any 3D application you will use from any company.

And of course some of these things might no longer work correctly because the current, old 3D feature set does not work properly with some current graphics hardware, so it is being discontinued.

@Samuel Kermit you haven't mentioned what version of Photoshop you're using but 3D features are been discontinued, more info

https://helpx.adobe.com/photoshop/kb/3d-faq.html

If you have the latest version of Photoshop and want to continue using the 3D features you can download version 22.2 from the Creative Cloud app

Sorry for spelling. *Do