Check your beef about 2-3/4 of the 3 hr mark. If it is 'fork-tender', take it out of the oven. If not, let it cook for another 20 min.
When meat looks/feels like it is almost ready to fall apart, take it out of the oven, put it on a platter, add all your veggies into the pan, check the water level. If you think it needs another 1/2 cup or so, add the broth from the pan the veggies were cooked in. (You want to have the veggies at least 1/2 way covered with broth) Put the meat back into the pan, cover it and let it finish cooking the veggies.