Just to get this right
You want to add a decal (little logo stuff) on a 3d object?
or just a decal on a screenshot of a 3d object? (so it won't be 3d anymore)
For the first, you need download permissions (and permission to use them professionally, since you are selling something) and a 3d program like blender (free) to add your decals as planes to the object
So for example like this
For the 2nd you need permission again to use their 3d model for your work, make a screenshot and place the decal in photoshop or illustrator.
Hope i got your question right