Creating a submit form- check box values into subject line

I'm creating a form that submits a filled out PDF to a specific email. I have three check boxes, if one is selected that value goes into the subjectline.

The code I'm using right now is:

var targetEmail = "name@name.com";

var subjectLine = "Material Determination Transition Form Received for Plant(s) - " + " " + this.getField("Check Box 21").valueAsString + " " + this.getField("Check Box 22").valueAsString + " " + this.getField("Check Box 23").valueAsString;

this.mailDoc({cTo: targetEmail, cSubject: subjectLine});
(function () {

When I tested it, I checked two boxes and the subject line reads:

Material Determination Transition Form Received for Plant(s) - Off 2100 2200

The unchecked box leaves a value of "Off" in the subject line. If anyone can help in either hiding the Off or to change the unchecked value of all the check boxes to a blank would be greatly appreciated. Ty!