You can free up a cross bar slot by making R3 your Limit Break, a button that becomes universally useful across all classes.

To start with you need to change R3 to be "Execute Macro #99" in your controller bindings.



Next you need to make macro 99 be Limit Break.



Finally, if you want to keep the previous function of R3 and zoom in to first person still, you want to turn on the following option in character settings.