每次打印前，再次对three声明

public class HelloWorld{

    public static void main(String[] args) {

    int one = 10 ;

        int two = 20 ;

        int three = 0 ;

        three = one + two;

       System.out.println("three=one+two==>"+three);

        three+=one;

       System.out.println("three+=one==>"+three);

        three-=one;

      System.out.println("three-=one==>"+three);

       three*=one;

       System.out.println("three*=one==>"+three);

       three/=one;

       System.out.println("three/=one==>"+three);  

       three%=one;

       System.out.println("three%=one==>"+three);  

}

}