My Three Years Old Labrador Has Fever, Loss Of Appetite And Pain In The Hip Region And Back Legs So That He Finds It Difficult To Sit And When He Does It's An Awkward Position. He Is Listless Too.Can You Help Him?

Fever can be caused by infection, pain, inflammation, or excitement.  Fever of unknown origin is a frustrating problem to deal with.  It can be due to an infection, since your dog also has joint or bony pain this could be an osteomyelitis-deep seeded bony infection. 
Musculo-skeletal pain could be the cause--I would rule out injury-like a fracture, dislocation, spinal cord injury or severe arthritis.
X-rays should be taken to search for bony changes associated with osteomyelitis, signs of fracture, and to get a good image of the painful area.  Bloodwork should also be run to rule out systemic infection and assess health of the internal organs.
Another cause of high fever, joint pain and lethargy is tick born disease.  Lyme, Ehrlichia, Rocky Mountain Spotted Fever, and Anaplasma are somewhat common tick born diseases--the prevalence varies depending upon area of the country.  I would have him screened these diseases.
Immune mediated diseases can cause joint pain and fever--this is where the immune system goes a bit haywire and starts attacking its own cells.
This sounds like a chronic problem and I do not think it will go away on it's own.  Please take your lab for an appointment with your veterinarian who can screen for the diseases discussed above as appropriate.

My 1 year old lab has the same problem. She started with it when she was about 7 months old. She has been to the vets on numerous occasions and has had several courses of anti b's but it just keeps on coming back. Did you ever get to the bottom of it??