Require fields when submit button pressed

Also, bear in mind that it checks automatically for "required" only when a form is submitted. This can be done with a Submit form action or the submitForm JavaScript. Nothing else. People insinctively expect that it applies when their "submit" action takes place (such as a save, a mailDoc call etc.) but no.

Use this and change field names to your actual field names:

var f1 = this.getField("Field1").valueAsString;
var f2 = this.getField("Field2").valueAsString;
var f3 = this.getField("Field3").valueAsString;

if(f1&&f2&&f3){
//put your script for submitting here
}
else
app.alert("Please fill in required fields before submitting file.",3);