The fit "automatically" doesn't work very well for me for the reasons you mention, and because there is a performance hit when Tableau resizes the dashboard.
The best layout depends on your users / use cases -- I often choose iPad landscape, if I can make everything fit, or letter-landscape if I need a little more room and will be generating PDFs from the dashboard. I've heard of places that use much bigger dashboards, because the users have big monitors are aren't using projectors or printing.
For your second question, I believe actions are executed in the order in which they are created. So if you created the action for tab3 after tab2, an action that affects both tab2 and tab3 will show tab3. Try deleting the action for tab2 and adding it again.