Ok, this is it:

You first reach the Hammer. So far, so good. With it, you have to do 3 Kills. As you mentioned. The first isn't hard. Just sneak up one guy from behind and press the left or right (doesn't matter) mousbutton once. That's how you do a "normal" kill.
For the 2nd: Sneak up another guy from behind, but instead of pressing the mousebutton once, you have to press it and hold it down and sneak the guy till the ring around the guy gets orange. the release the mousebutton and you do a gruesome kill.
3nd: Sneak up the third guy and do exactly the same as before, but instead of holding the mousebutton till the ring gets orange, hold it till it gets red. So you do a violent kill.
that's it. After the "Hammertime", you have to do the same with, I think it was, a Baseballbat. But it'll be explained again by this guy in your ear.

good luck, mate :thumbup:

__________________