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If this is not working after three years then it may not be worth it. You may want to try a glucosamine in combination with chondroitin sulfate supplement. This seems to be more effective for some dogs.
Here is dosing information
You may want to ask your vet if they feel that a medication called Adequan would be helpful.
Here is more on this medication.
I hope this has given you some additional options to discuss with your vet