I would ask myself "why does an account have to be unique by these three attributes?" and then "how can i restructure the table so an account has its own data that does not require the triumvarite unique attribute?"
I dont know your data or problem domain so i cant provide specifics to your exact situation. perhaps we (the denizens of this forum) could provide better suggestions if you gave some details.
While this question certainly can be taken further, as coherentkris says, you can start by having your auto-enter field use a formula along the lines of:
not IsEmpty ( Type ) and not IsEmpty ( Year ) not IsEmpty ( Month ) ;
Type & Year & Month
Tank you for having a look
That works, brilliant