our labs are attacking our yorkie/terrier cross

Question
We have 2 labs, a black male 2.5 years old, and a yellow female just under 2 yrs old. We also have a yorkshire/terrier cross. All 3 dogs have lived and played together happily without any major problems. A few times the male lab has grabbed the yorkie by the cuff and tossed him but never using teeth and it was always when the yorkie grabbed his toys. We moved about 2 months ago, but they were still all fine together until 4 days ago. We went out of town leaving our dogs at home with the care of a pet sitter. While we were gone the pet sitter informed us that the female attacked our little dog with intention to kill. The little dog has wounds to prove this. The pet sitter did say they were plying with a stick and the little one went for it. Well today I had them all outside and the male lab attacked the little dog grabbing him by the throat surely wanting to kill. He was rag dolling him and at one point the little dog went limp and I was sure he was dead. Why are they being aggressive towards the little dog when they have lived together for 7 months without this serious of a problem. Help me please, I'd hate to have to get rid of any of them!

Answer
This may not be a problem that can be fixed over the internet.  somebody that can recognize what is happening may need to observe the dogs.  There is some really subtle stuff, body position, sniffing, bumping, etc.  If the terrier mix is less than 3 years old, it may be challenging the Labs for pack rank.  I remember a Husky challenging a Lab that weighed twice what he did.  

first, I would spay/neuter the lot if not already. Then work on obedience.  You need to be top dog, and the rest competing for third place.  The key to most behavior problems is approaching things using the dog's natural instincts. Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat.

For now, don't leave the terrier with the Labs unsupervised.  If you are not using crates, picking one up for the terrier may be a good idea.  

You may need to get a private trainer or behaviorist.