My 3yo has what looks like a scarlet fever-like rash. She had a fever last week, but hasn't had temperature in 5 days. She has had a cough. Er or md?

MD. Since she doesn't have a fever now and her only symptom is a rash, i'd recommend seeing your doctor. Doesn't sound like an emergency. Sounds like she has a viral exanthem - a rash associated with a virus. If there's more to the story - is very lethargic, has a headache, is acting abnormal - then take her to the er.
MD. There's no rush for the visit, as long as she is drinking fluids and not inconsolable. Continue encouraging her to drink fluids and get an appointment next available.
See your local doc. This does not sound like an emergency, and a call to your doctor's office will establish how rapidly they want to see your little girl. Good luck. Hope she is better.