Question: how long to cook a 3 pound roast at 300 degrees?
Answer: Preheat oven to 300 degrees F (150 degrees C). ... 
Place roast in a large baking pan; arrange potatoes, onion, carrots, and celery around roast. ... 
Bake in the preheated oven until roast is browned and cooked through, about 3 hours. ... 
Reduce oven temperature to 200 degrees F (95 degrees C).