I would like to cut my metal front door by 3 inches,how do we do so and put the bottom of door on,as the door frame is easy

Contact Stanley, and have the model number of your door available. The manufacturer will advise exactly how much, if at all, the door can be cut. I believe metal doors have a certain allowance for minor cutting. Whether it can handle 3 inches is a question for the manufacturer, who will be happy to help. You might look to see if Stanley has a website...the answer might be right there.