Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Thrown knives need to be ducked under; slashed knives need to be jumped over.
Don't focus on one assailant - keep moving and attacking in all directions.
If time's running out, whip out your gun to finish the bad guys off.