Breast distortion on u/s stable if not smaller after 3 months... Is this likely benign then? Still nursing my son.

Keep checking. The fact that is was the same or smaller is always good. Plus you are young which is in your favor. Nursing caused swelling of your breasts and thus makes it more difficult to evaluate. A referral to a good breast center would be warranted. Dont let nursing stop whatever you need such as mammogram or mri, aspiration, biopsy to be done.