Hi cyril sun,
This can be done simply using Auto Refresh widget and Navigation widget. And bind the refresh event of Auto Refresh widget to navigate of navigation widget.
Auto Refresh widget: refresh interval as 30 seconds
navigation widget: target window as Replace
Set the visibility as false for both widgets.
Select the MashupName to which the Mashup should navigate after 30 seconds.
Repeat the steps for all 3 mashups.
I hope it helps.