Combine The Three Values Into A String, Create A Md5 Of The String, Then Call The Value ?

I have made a basic form, and I need to combine three values within my form, then create an md5 hash of this string.Then assign it to a hidden variable.My form is here...

Code:
<p>
<label for="firstname">First Name: </label>
<input id="firstname" type="text" name="firstname" /><br />[code]....

Or I have created a pastebin of it here, for easy reading: http://pastie.org/1171757.So I need to be able to combine the three values into a string, create a md5 of the string, then call the value of the string into a hidden value all before posting the form.