I assur you that Crossfire will onyl allow 2 monitors. They must be pluggted into the primary card. Despite knowing this first hand having owned a couple crossfire rigs, you can loko it up on wiki easily enough.
Now, as for using a 3rd monitor with the same two cards, that you can do.. but you must disable crossfire first, when that is done the second card will act on its own and allow you to use its outputs. You can set up a profile to switch quickly. But make sure the drivers are up to date or crossfire enable/disable can be buggy.