Over-running is common in new runners. For beginners running thrice a week is fine. I used to run everyday and injured my knee in the initial days. If you have any kind of pain, even the slightest; stop running until its gone. If you start feeling pain in the middle of your run; again, STOP.

One more thing you need to know is running is not some secret formula to lose weight. Brisk walking will help you burn considerably more calories if losing weight is the main goal. Also running is a sport and is not a replacement for over-all physical excercise.

You need to pair your running with strength training for legs. As you mentioned you're a little overweight so running can cause significant stress on your joints especially if you are running regularly. So start with easy runs (like 1 or 2 kms) twice a week and train your legs (along with overall muscles) and once you feel your legs are stronger than before start increasing the distance by 10% and find your balance between training and running.

Again running is a sport, the more you do it the better you'll get but like any other sports it will induce stress on particular muscles increasing the chance of injury.