I'm not 100% sure about it, but from what I've read on Wowhead comments, you need to kill the guardian dwarves as soon as they get hit by Razorscale's flame breath.
Again, I'm not 100% sure about it, so don't take my word for it.
That might be it. Although I don't remember killing any of them personally so the 3 I got are still a mistery. It was my first time in Ulduar too, so it couldn't have been from before.
Gonna try again with my ret pala alt now. I'm only missing 3 achis for the 10 man, including the mentioned one.