It takes 9 to 10 hours to cook a 3-pound beef top round roast in a Crock-Pot at the "low" setting. The roast only requires about 5 hours of cooking time at the "high" setting, but this tough cut of meat isn't recommended for cooking on the "high" setting. The faster cook time and higher temperature doesn't break down the tough meat fibers and leaves the roast tough.
Beef roasts have greater flavor when the outside is browned before slow cooking. A slight sear seals in the juices to help tenderize the roast. Dust all sides of the roast with flour and cook the roast in a skillet with hot oil for about 2 minutes per side or until evenly brown.
It helps to start the slow cooker on the "high" setting just to bring the liquid up to temperature faster. Add the seared meat, spices and liquid such as water or beef broth, and aromatic vegetables like carrots, celery and onions. Reduce to the "low" temperature setting when the liquid comes to a boil.
The trick to Crock-Pot cooking is to keep the heat contained inside the pot. Set the timer for 9 to 10 hours and leave the Crock-Pot undisturbed until done. If the lid is removed, heat escapes the pot and it must return to the temperature. If potatoes are desired, add them about 2 hours before the end cook time, to avoid overcooking but replace the lid quickly.
A meat thermometer is a more accurate way to determine if meat is done than relying on a timer. Beef is considered well done when a thermometer at the thickest part of the roast reads 165 degrees Fahrenheit.