Priority is used to set whether a Layout will Display if another Layout is scheduled to Display at the same time. For example let’s say you have 2 Layouts, one with a Priority of 1 and another with a Priority of 2. If you scheduled those Layouts to display at the same time, only the Priority 2 Layout will appear, as it has a higher Priority. If they have the same Priority, they will both play.
Display Order is different as it sets the order that Layouts will display in if they are scheduled at the same time. For example again you have 2 Layouts, one with a Display Order of 1 and the other with an Order of 2. If these Layouts were scheduled to display at the same time, Order 1 Layout would play followed by Order 2 Layout.
In your message you mentioned having 3 Layouts, all with a Priority of 1 but with different Display Orders. As the Layouts all have the same Priority, they will all display if they are set to show at the same time. The order they will play in will be set by the Display Order, so Order 1, then 2 and finally 3.