I think what you are talking about deals with pageflow, not just conversation. You can define a pageflow using JPDL language, with every navigation possible in your pageflow. Then you start a conversation with this page flow and Seam will check any navigation, which is what you need if I understood well. You should have a look on the numberguess example bundled with Seam distribution.
Assuming that you're 3rd pages *.page.xml doesn't have a <begin-conversation> (or a conversation started in any other way) then you can specify a no-conversation-view-id. That should redirect your attempt to directly access the 3rd page.