combining information from several form fields into one attribute

Hello,

I would like to use three text boxes, one for each section of a social
security number. I've been looking into the date time select which
essentially does the same thing, except with combo boxes. The
resulting html looks like:

<select name="employment_record[termination_date(1i)]">
..

<select name="employment_record[termination_date(2i)]">

..
<select name="employment_record[termination_date(3i)]">

Is there helper that accomplishes what I want to do with text inputs?
What is the significance of the 'i'? Do I need to roll my own?

Thanks,
Dan