Turn the heat down to 225 or 250. If you want your chuck roast to fall apart similar to pulled pork, you have to take the internal temperature of the meat up to around 190-195, in order to render all the fat and connective tissue. At 300 degrees, such a small roast will start to overcook before those tissues get a chance to melt away. At 225 to 250, you're giving your meat a fighting chance to be edible. Even still, 3 hours may be too much time. Check it at 1.5 hours, then every half hour or so afterwards, until you reach your desired effect. Like I said, for pulled beef you're probably taking it up to 190-195. If you want to slice it, take it to maybe 175. But don't let time and temperature be your only guides -- use your eyes, fingers, etc. Test for tenderness with your probe, and make sure it doesn't look like it's drying out.