Is a rash on my breasts after bilateral mastectomy with expanders normal 3 weeks post op?

Call your surgeon and notify him or her of these changes.  Your surgeon will want to know of any changes in your incisions especially with expanders in place. Best wishes. 

While this sounds most like an allergic reaction (either to tape/glue if there is any on your incisions) or possibly a reaction to one of your post operative medications, you should call your physician's office to ensure that this is not something more serious like an infection.