is there any easy way of implementing two feature 1) auto tab and 2) allowed only numbers

i have a SSN box below as soon as the user enter three digits will skip to the next .. so on and so forth.....

    <TR>
    <TD nowrap><b>Social Security Number</b></TD>
    <TD align="left" nowrap colspan="4">
<INPUT NAME= "SSN1" TYPE= "TEXT" SIZE= "3" MAXLENGTH= "3" VALUE= "">
<INPUT NAME= "SSN2" TYPE= "TEXT" SIZE= "2" MAXLENGTH= "2" VALUE= "">
<INPUT NAME= "SSN3" TYPE= "TEXT" SIZE= "4" MAXLENGTH= "4" VALUE= "">
</TD>
    </tr>

also i wanted to restrict to have only numbers.