When I make a Arm or Chuck Roast always after 5 hours it is still tough, the beef is grass feed. I have give up making them. My wife wants me to make another Arm Roast again and I want it to come out month watering and not tough. My wife has ALS and it really needs to be very tender for her. I would like to make it in a Dutch Oven in the oven, I would set it at 250F but how long for a 3 pound roast? I would like it to be very tasty with a sauce or gravy.
If it's too tough, keep cooking it. Some people use "tender" to describe a pot roast that is tender like a good steak, others want it to fall apart with no knife required. I regularly cook pot roast 8-12 hours. As log as you have it covered for most of the time (like in a crock pot or in the over covered with foil), it will keep getting more tender until it falls apart under its own weight (which sounds like it may be what you want).