You should look into enrolling your dog into school, they teach the basic commands along with plenty of socialization. Go to Pet Smart. There classes are $109 for and 8wk class, and its so worth the money. The trainers are very knowledgeable and willing to work with you and your dog to get your dog help.
At 3 the dog is past the initial (puppy) training, but may still be young enough to be socialized. Depending on your financial situation, a private trainer who is experienced with your breed and this dog's problems might prove very helpful. Training would prove less expensive than vet bills for another pet your dog might injure.
There are several excellent breed specific forums out there. You might do well to find one (sorry I can't provide a link to any, forum rules) and talk to other Mastiff owners, who may have some more specific advise.
***Edited By: pyrmom on 8/8/2009 10:30:21 AM*** Reason: *
"Some days you're the dog; some days you're the hydrant". author unknown