If two boxes are checked, third box will auto check

The problem here is that a checkbox does not have a custom calculation script that would allow you to determine if the checkbox should be checked or not, so we need to find a different way to accomplish this. I often use a hidden text field do perform this task for me. Another solution would be to write a document level function to update the checkbox, which gets called on any click action on the other two checkboxes. 

I will use the first approach here. Create a text field and mark it as read-only and hidden. Create a custom calculation script for the text field and then use this script:

// only check "Check Box3" if both "CheckBox 1" and "CheckBox 2" are checked.
console.println(this.getField("Check Box1").value);
console.println(this.getField("Check Box2").value);
console.println(this.getField("Check Box3").value);
if (this.getField("Check Box1").value != "Off" && this.getField("Check Box2").value != "Off") {
    this.getField("Check Box3").value = "Yes";
}
else {
    this.getField("Check Box3").value = "Off";
}