How long do you cook a 3.46 lb Corn beef brisket at 300 degrees?

The general 'rule of thumb' for cooking meat is - 20 minutes per pound PLUS 20 minutes. In this case, the cooking time would be 5 hours 40 minutes. Towards the end of the cooking time - pierce the joint with a skewer. Press the flesh lightly - if the juices run clear - it's cooked thoroughly. If the juices still show blood - cook it for a little longer and re-test.