It's hard to suggest anything for something that's already very good.

The eyes of the right face aren't even with each other. The lighting is a bit odd to me in places. For example, the center face has a highlight on the top of his head and brows but also on the bottom of his temples. That seems to be reflecting off his brow, which is believable because of the metallic look of the face, but I don't think the highlight should be that strong there. The cheek bones also have a similar highlight on the bottom of them but in this case there's nothing to really be reflecting that light onto them.

The lighting on the end of the nose for the center face seems too low. Because it reaches its brightest at the very bottom edge of the nose shape that suggests either the nose is pointing downward or being seen from above. I would raise that bright point to be slightly above the point of the nose that's closest to the viewer if the light is coming from above.