Stopping a submission
Here is a segment of a fillable form I've created.

It is necessary for the user to use the check boxes to select three days. I have a piece of javascript that that creats an alert unless exactly three days have been selected. The problem I have is that I cannot stop the form from being submitted. It is not like I can make the fields required, because at any time two of the checkboxes will be blank.
So how do I stop the submission?
Thanks - Jack
