buyer denis depending discount group item items products purchased purchases total vary volume

how do I set up a volume discount for a group of my products (not all items) so that if a buyer purchases say 2 of item a , 3 of item b and 4 of item c, I can give them a discount based on the 9 purchased - discount will vary depending on the total purchased

Denis

Sorry, but that is not possible to do with E-junkie