How to be markup LABEL element with two or more fields.

Dear all,

I am wondering why LABEL element's FOR attribute value is not 
IDREFS type. I want you for me to think the following scenes. 
For example, when inputting a telephone number for every hyphen:

Tel. [        ] - [        ] - [        ]

If the markup of this is carried out using LABEL element, 
what does it become?

I want to come to markup in this way as below:

<label for="tel1 tel2 tel3">Tel.</label>
<input id="tel1">-<input id="tel2">-<input id="tel3">

However, since FOR attribute value is IDREF type, this is invalid 
in HTML 4.01. What is an appropriate markup in a case like this?

Thanks,


Best regards,
Yuu Morita