3rd Lab in our house

Question
I currently have 3 year old son. We have a black lab/ golden retriever mix that has just had his 1st birthday. He has been neutered. We also have a English Choc lab that is 7 years old and is also neutered. Both dogs have been extremely good and are great around my son.
    I have just adopted another female American yellow lab that is four years old. She also has been spayed. My best friend has major family problems and she had to find a (temporary) home for lab. I didn't have the heart to let dog go to someone else especially since it probably will not end up being temporary.
    Before she came to my house she was a pretty hyper dog. I believe it was because she didn't get a lot of exercise and attention. Her situation didn't allow her to be just a dog.
    The first week at our house we made sure to spend a lot of time with her. We introduced her to the other dogs slowly but we also took her work everyday. Her behavior did a 180 and she became a much more mellow dog. In fact, my girlfriend couldn't believe that she wasn't jumping on everyone. I believe all three dogs are getting along and do enjoy each other company.
    My issue is with my young lab. Last night I saw him snap at my son when my son was trying to pet him and give him a hug. My son was very calm, had put his hand on him very slowly and gently. My son then leaned over to give him a hug and that is when he snapped at his head.
   My first thought was that maybe the dog had been injured because he has never shown this type behavior. I had looked him over and I didn't anything. I then immediately had my son do the same thing pet the dog in a calm and gentle manner and then give him hug while I was sitting there. The dog was his normal self again.
First question is do you think the third dog could be part of cause with younger dog's behavior?
Second did I handle that situation correctly? I wanted to try and make sure that my son wasn't afraid and the dog learned that it ok for my son to hug him. Should have waited instead of getting them together right away?  

Answer
I think what you did was right.  I also think the new dog may have upset things.  This whole thing may call for strong leadership on your part.  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. Start at http://www.dogsbestfriend.com/ For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm  They have some additional material on dogs and children.  Read through it.