How much weight can I loose in 3 months by walking 30 minutes on treadmill at 4.7 km/hr or by running 30 minutes on treadmill at 7 km/hr?

Not Sure. I'm not sure about the weight loss relationship to your walking or running, but I am certain that the correct amount of exercise and the type of exercise best suited to your needs is whatever you will enthusiastically stick to over the long run. All too often people "burn out" chasing an exact target of minutes and speed.