I want to stay after the date of '' enter before'' I have valid visa and want to enter China 3 days

Duration of stay and enter before date are not the same thing. You can enter China any day before your "enter before" date, but your duration of stay is whatever indicated on your visa and will not be affected by the enter before date. To know when you have to leave, add your duration of stay to your entry date and that's the day you have to leave China.