3 wk post op, I am worried that my nipples will permanently be facing downward at the bottom of my breast and not in the middle.

When we refer to implants dropping, we actually refer to a relaxation in the tighter lower part of the breast. This 'lower pole' will give and relax to accommodate the implant and your result should improve, and this will take several weeks. We like the use of a breast band over the top to encourage the process.

In general, your implants will drop to a point.  If you had smooth implants placed, these should be more mobile and your surgeon will likely ask you to begin massaging them.  The likelihood of having your implants drop will be higher.  If you had textured implants, they may not drop to the degree you may want.  The capsule adheres to the implant and they generally stay fixed in that position.  Speak with you plastic surgeon to ensure you have a proper plan in place.