My muscles got hurt in shoulder and top arm while lifting a gate. Arm is in sling, very painful and almost 3 weeks like this, not healing. 2 doctors have no idea what the problem is. Can I use deep heat rub and bandage and put arm back in sling? Any pain killers that will help for the pain?

The standard tylenol/advil will work for pain. It will take awhile for the muscles to heal depending on what you did to them. Some won't be able to be fixed out side of surgery. You can use whatever you want, but I can't say if it will have a permanent effect. Most of that either promotes healing or just relieves pain.

I would go see if you can be referred to a specialist. Make sure you keep the arm in the sling.

Some minor shoulder pain can be treated at home. Icing the shoulder for 15 to 20 minutes, three or four times a day for two to three days can help reduce pain. Use an ice bag or wrap ice in a towel (putting ice directly on your skin can cause frostbite).
Other home treatments include:


resting the shoulder for several days before returning to normal activity and avoiding any movements that might cause pain



using over-the-counter nonsteroidal anti-inflammatory medications to help reduce pain and inflammation



compressing the area with an elastic bandage to reduce swelling

The issue, is that anything minor, should be getting better by 3 week. So there is some underlying, issue or problem.

I would say that another doctor, perhaps even consider some therapy.

I am in China, here I would say try acupuncture and massage treatment,

And of course, codeine is over the counter here.