For 3d, to take Blender as an example, a zero-level diff feature would be to export (e.g. as glTF) and show which scene objects differ (meshes, textures, lights, etc.) More detailed parsing would give more details, e.g. image-diffing modified textures.

As I noted, merging is nontrivial. But it seems important -- if I save a branch, go back to the trunk and do more work, how do I add in the changes I made on the branch? Or even know what those changes were? Good diffs would help of course.

Merging would certainly have to be product-specific. For some open-source tools one could imagine hooking into their undo/redo logic, or parsing the product files into a temporary line-based format, but from outside it seems much more challenging than line-based merge in git (which is already imperfect for many applications).