I think you might get further doing some one on one training with him without the other dogs to scoop treats or take your attention.
You want him to take his cue from you about what is or is not dangerous. Right now he feels he has to fend for himself when dangerous ie strange humans aproach. You want him to follow your lead about this instead of making his own judgement calls.
It is good that he chooses to remove himself from the danger rather than trying to drive the scary people off. Backing him into a corner on this issue is not a good idea.
He may actually be so used to being fearful of strangers that you would be best to work with a veterinary behaviorist and use a medication to take the 'edge' off for him so that he doesn't shut down to where he cannot learn a new behavior. It is nearly impossible for a dog flooded with fear hormones to learn something different.
Another approach would be to determine at what distance he starts getting anxious and work with him at that distance on obedience commands with rewards for right behavior.
This site has good info on desensitization
http://www.clickersolutions.com/articles/2001b/desensitizing.htm
you can read some here on the calming signals they discuss
http://www.diamondsintheruff.com/calmingsignals.html
I actually would teach my rescued danes, and still do with my other dogs, a 'command' called 'say hello' what Say Hello sounds like to humans is this is a trained friendly dog, what it means to the dogs is - this is a friendly human let them touch you and you will get a treat from me for that behavior. Its a command you can practice with people the dog does trust like your partner and can be used on you when your partner holds the leash. Eventually you may be able to work up to using it with strangers.
With young dogs I'm raising I actually also play a 'grab game' where I grab a body part and when the pup turns I stuff a treat in the mouth. I want my dogs as bomb proof as possible in case some child grabs them. This may be far too overwhelming for this dog but might help another puppy in the future.
Bear in mind your dog has had three years to perfect his avoidance and fear behaviors so anything you do to change them will take time.
You may want to try clicker training and this site scrolling past the ads has some good videos on it
http://www.clickertrainusa.com/clicker-training-videos.htm
For getting the dog used to viewing you as the leader I'd suggest the nothing in life is free program which is totally non violent
http://www.paws.org/cas/resources/fact_sheets_dogs/nothingfree.php
and people often see progress in a week of using it.
But this may be more than you can handle on your own. Ask your vet about a behaviorist or local veterinary behaviorist that can help you with the problem.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics