異なる符号化TXTファイルの最初の3文字は異なる

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

public class EncodeTest {

	/**
	 * @param args
	 * @throws IOException 
	 */
	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		InputStream is = null;
		String[] filenames = { "d:/gbk.txt", "d:/utf-8.txt" ,"d:/unicode.txt","d:/unicodeBig.txt"};
		for (String filename : filenames) {
			is = new FileInputStream(filename);
			System.out.print(filename+"    3   (HEX):");
			for(int i=0;i<3;i++){
				int ch = is.read();
				System.out.print(Integer.toHexString(ch));
			}
			System.out.println();
		}
		if(is!=null){
			is.close();
		}
	}
}

 
 

d:/gbk.txt    3   (HEX):616263
d:/utf-8.txt    3   (HEX):efbbbf
d:/unicode.txt    3   (HEX):fffe61
d:/unicodeBig.txt    3   (HEX):feff0