Keybinding with a mouse

I'm playing a game (BF4)

How do I assign LMB, RMB and MMB as a keybind?

How do I unassign aswell?

(e.g. If I wish to remove SPACE (instead just use LMB as fire) as a keybind i.e. just blank)

Basically, I want to use SPACE as 'push forward' and LMB as fire (when flying). (not both of them for fire.

Hope that makes sense

Thanks

PS - when I click LMB as a keybind nothing happens - am I doing something wrong?