Pain in the same distribution 3 weeks after a microdiscectomy is not necessarily unusual. The compressed nerve can swell after the pressure is taken off and the swelling can take some time to dissipate. If that is the case, an oral steroid can be effective to reduce the swelling as long as you understand the potential side effects of oral steroids.
Pain in the opposite side is somewhat more unusual. Inflammation in the canal can affect the opposite nerve. If there is a post-operative hematoma (a small collection of blood that causes some nerve compression in the canal), that can cause this pain. A recurrent disc hernation can occasionally cause this type of pain. Infection can also produce these symptoms but most likely, significant back pain would also accompany an infection.
You need to contact your surgeon for further follow-up.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.