Cronic diahrrea for about 3 months now, been to 2 doctors, no solution. What can be done?

Thorough evaluation. Need comprehensive history, including family history, personal travel, work, diet diary (including sugarless gums, candies, dairy, wheat, etc), antibiotic use; thorough physical exam; appropriate labs; maybe stool tests, evaluation of colon, upper GI and small bowel. Most cases can be pinned down.