ADD--->Mama Tex I believe the name was has all the accurate information. The only thing to be concerned about as I said below if the truffles have high concentration of dark chocolate. Still be warned though, because your dog is going to have a very bad stomach ache, and you don't want to make it worse. That's why I suggested checking the gums and such and keep him confined so he can't run around and keep getting sick and will stay hydrated.<---
Depends on the type of chocolate and exactly how much. Dark chocolate contains much more chocolate than milk chocolate. Eating three pieces of small dark chocolate truffles is going to be more lethal than milk chocolate.
My vet has always said it being lethal is something like 1lb of chocolate to 20lbs in a dog. I'm sure someone will correct me if I'm mistaken (my brain is pretty scattered today) and will give you accurate measurements. But every type of dog is different.
Your dog is only 12lbs, I think this might be a very bad thing. Feel around your dog's abdomen, is it bloated? Check your dog's gums they should be nice and pink, if they're getting very white then your dog could be getting dehydrated. Don't make your dog throw up anymore. Try and keep him laying down or confined, put him in his crate or a playpen or something if you have to, and keep him close by to monitor. You'd be surprised how quickly something can come on while your dog runs off into the next room!
Try your best to hurry into the vet, if you absolutely can't call an emergency hot line and ask them for help, they'll tell you the best things you can do, and will ask you questions that will help assess your dog's condition.
Good luck!