Hello dear ,
Fetal renal pyelectasis ( kidney swelling) is a cause for worry only if it is more than 7mm in anterior -posterior plane at more than 28 weeks of pregnancy. So , in your case , if it is 3 mm , then it should be normal . If you are still worried , then a post delivery baby ultrasound of kidneys will crosscheck it and reassure you . Hope that answers your query .
Thank you

Depends on the degree of swelling of the kidney of the baby. It may improve with progression of pregnancy in approx 50% cases . Has a little bit association with aneuploidy as well. Will need follow up and post natal ultrasound and evaluation.  Follow up with your obstetrician/ fetal medicine specialist to see how things progress.