为什么three前不能加int？

public class HelloWorld{
    public static void main(String[] args) {
	    int one = 10 ;
        int two = 20 ;
        int three = 0 ;
        int three=one+two;
        System.out.println("three="+three);
        int three+=one;
        System.out.println("three="+three);
        int three-=one;
        System.out.println("three="+three);
        int three*=one;
        System.out.println("three="+three);
        int three/=one;
        System.out.println("three="+three);
        int three%=one;
        System.out.println("three="+three);
        
        
        
        
        
        
        
        
        
        
        
    
	}
}