这三个疑问句有何不同？

有一读者问：下面几个句子有何区别：

(1)Whom are you writing to?

(2)Who are you writing to?

(3)Who are you writing?

上述句（1）只用在很正式的场合或书面语中；句（2）最为普通；句（3）只用在口语中。