How to Edit 3D textures?

3D minecraft resource packs are "3D" because the .json files for their models are customized. In the resource pack folders there will be a textures folder and a models folder. Move the models you want and move the textures you want but do note: The textures may be specially done and so will not always work with normal textures. Always compare the texture you want with the one for the model and if they are different then you either use the unwanted texture or don't use the model.

Locate the model\block\[file.json] file in your resource pack that you would like to keep, copy it to your desktop and open it.

Under the tag texture or texture find all the image textures this model requires to function.

Locate those images under textures\blocks\[file.png] or wherever they could possibly be found and copy all of them to your desktop.

Open your resource pack that you want these files to move to and replace the files already there with the same name, both the .json and the .pngs.

This should lead to the full model and textures working properly in the new resource pack.

(Notice: This method will replace all of the blocks that rely on the textures you copied. For example, if you changed the acacia fence gate model and texture, the standard plank block will also have it's texture changed.)