In my experience, once you kill one of your attackers, you have to type 'kill <whoever>' to continue fighting. It's basically as if you just typed 'stop attacking', unless one of the others types 'attack xeethot' again (then you see the message, 'blah blah advances on you with his supremely accurate indestructible dildo of vanquishing' or whatever), in which case you'll defend yourself as per normal. You still dodge, but postures don't come into effect anymore (AFAIK), so since you're not on defensive you get slapped around more.
So always attack the next dude. And change your posture after.