The first one's robotic parts are good. But those lines are messed up there.
The compotition of the face is a little bit too small.

The second one looks not bad. Those eyes are too small, make it bigger.
That nose is a little bit weird and those teeth are messed up too. Add more shading will make it look better, I guess.
Those shading on horns are good, need a little bit edit and It'll be perfect.

The last one looks.. Cute actually.
That eyes tho, rotate it to "V" will be a lot more sexier.
Those words at the back of head is not needed. Delete it will be better.