You can use blender 3D to export your embedded textures as seperated files by loading up the model into blender. It doesn't matter in which format the 3D model is either its a fbx, 3ds or obj file it doesn't matter.
just import the model into blender and follow the process shown in rest of the tutorial.
There can be different ways to export textures from the embeded model using blender but today in this tutorial we are going to see 2 methods to this, one is to simply save the texture as a image file and the second is to pack and unpack .blend
Assuming that you have already loaded or imported the model file into blender and it has textures applied to it.
Now just change the viewport from 3D view to UV/Image editor as shown in the picture down below.
Now select the texture you want to export there can be many textures depending on your 3D model for example Normal Map, Diffuse, Specular, etc. Select the one you want to save as shown in the picture below.
Once you have selected the texture you want to export then go to image tab and select save image and then save the image file to desired location as shown in the image below. That's it you have your texturesm, now go ahead and import them to your game engine Like Coppercube, Unity or in whatever program you want to.
If the above method of saving image file doesn't work for you then you can always use the second method of packing and unpacking of blend files. Load or import your 3D Model into blender. First you need to pack the file, then go to File > External Data > Pack All into Blend.
Then you can unpack the files to unpacks files go to File > Externala data > Unpack All into Files.
this will export all the files used by the 3D model to the Folder. Always choose the option "Use file int the current directory" while unpacking so that all the files goes to the same folder. See the picture Below to get idea about how to pack and unpack file
Blender version 2.79b was used while writing the tutorial if you want you can use the latest blender 2.8,
Also use blender renderer as your renderer not cycles or eevee renderer.