Nice penciling! The third one looks good. But be careful to not over exagurate your gesture or just your lines. In the second skull there is too much points and diagonals on the face, like the super pointy and sharp chin. The first skull looks good but it seems too square, specially on the upper part of the head. You could use 3D online models to practice anatomy, one site I reccomend is sketchfab.com, there is a great library of simple 3D models. You already seem to master the planes and it's shadows, so keep up the great work!
i love skulls so much idk why 😂 i love looking at theme and i love drawing them a lot. and these all look incredible. i also love the fact that you drew all of them with what appears to be a red pencil i also really like the different angles of the skulls all to gather beautiful. keep up the awesome work @Valerio Pilotti ✌😉
Try to use one line as an outline for the skull insted of doing the thick scracky ones . And try not to add light and shadow untill you have the shape right.