Hi, I understand your concern. These symptoms are common especially in the third trimester of pregnancy. The abdominal pain that is causing discomfort is probably due to heartburn that effects pregnant women due to growing uterus, pushing the stomach upwards and forcing the stomach acid into the esophagus. Back pain is caused due to the extra weight the spine has to support when the women is pregnant. This low back pain is aggravated in the third trimester due to hormonal effects that cause relaxation of the ligaments supporting the joint around the pelvis. Though, constipation is a very common complain during pregnancy, constipation for three weeks is of some concern. If you have already tried increasing fluid intake (water, fruit juices), high fiber diet (fruits and vegetables) consumption, and moderate physical activity (walking), then you should visit a healthcare provider and get examined.