I am setting up a form to enter a chat queue on my website for work. The form script comes from the company we are using to host the chat, but the problem is that customers don't have to fill out anything to enter the queue. I want the users to have to fill out 3 of the 5 fields before they are able to click the sumbit button.
I've found a few scripts on the web that looked they like would work, but they were all for radio selections, and I don't have enough javascript knowledge to switch the correct paramiters to work with imput.

This is the script I am using;
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<form name="channel00000" action="https://secure.logmeinrescue.com/Customer/Download.aspx" method="post">
<tr><td>Please enter your name: </td><td><input type="text" name="name" maxlength="64" /></td></tr>
<tr><td>Company: </td><td><input type="text" name="comment1" maxlength="512" /></td></tr>
<tr><td>Phone: </td><td><input type="text" name="comment2" maxlength="512" /></td></tr>
<tr><td>Username: </td><td><input type="text" name="comment3" maxlength="256" /></td></tr>
<tr><td>Error: </td><td><input type="text" name="comment4" maxlength="64" /></td></tr>
<input type="submit" value="Submit" />
<input type="hidden" name="EntryID" value="00000" />
<input type="hidden" name="tracking0" maxlength="64" /> <!-- optional -->
<input type="hidden" name="language" maxlength="5" /> <!-- optional -->

I need the name, comment1, and comment4 sections to be filled out before the submit will go through. I don't care if the button is not available until they are filled or a popup that asks for them to be filled out, just so long as they can't enter chat without us knowing their name and whatnot. From what I understand, the bulk of the form script needs to stay the same so it directs to us and not another company, but other than that, I'm open to any suggestion.

Thank you in advance, I need all the help I can get.