It has been three days since I've had a child. Should I still have water retention in my feet? What should I do?

It depends. If you had severe swelling before delivery, it may take time; this also depends on your weight. If you are a healthy weight, lying down for 30-60 min 3x/day with your feet elevated can lead to rapid resolution. You should also walk fairly vigorously 3x/day. Have your blood pressure checked to make sure it is normal and not worsening. The edema could be a sign of bp/pre-ecclampsia.
Give it time. You have spent the last 9 months retaining water in your legs and feet. I won't go away in just three days. Give it some time and mention it to your doc at your first post-partum checkup.