I've stumbled across the options in Administer - System Settings - Option Groups and I read somewhere that you can create new ones.. so I tried using the syntax of existing options:
eg Previous 2 calendar years = "previous_2.year"
so I tried Previous 3 calendar years = "previous_3.year"
but that didnt work- just returned empty results (tried lots of others and they all did the same)
any clues please?