Dennis @ PerfectForms
I can think of two solutions:
1) Use a Multiple Branch object with three Notify objects. On the join paths from the Multiple Branch specify your conditions (Email 2 is not empty AND Email 3 is empty – send notification to 1 & 2, not 3, etc…)
2) Use a hidden helper field. When the submit button is clicked set the hidden helper with a formula (email 1;email 2;email 3). Then set the role to which you are sending the notification to the hidden helper field. No spaces between ; and next email.