Cook the meat longer and slower. Most folks cook anywhere from as low as 185 degrees to as high as 225, but 300 degrees for a brisket is just too hot. Naturally, when you lower the temp, you have to lengthen the cooking time.
Most folks say one to one and a half hours per pound of brisket.
You say you are cooking about a 3 pound brisket, which is pretty small. Most briskets will run from 8 to 12 pounds, so you've got some really small ones. Don't trim off the fat until it has completely cooked. Each time you open the cooker you lose heat and the cooker needs time to build up heat and readjust itself again. Keep experimenting, and it will get better and better with time.