buy 1+2 and get the price difference calculated in item 3

Hello

we have the following requirement:

Scenario1

if I buy item 1 and 2

and item 3

the following should happen :  3-(1+2)

eg item 1 price 10, item 2 : price 10

if i then buy item 3 (eg with price 100): the logic would be to pay for item 3 : 100 - 20 = 80

--> i did setup a formula for this

but when I enter the order

and put in item 3 : it automatically applies the value, of 80

without having defined item 2 , and 1 on the order

which should be the prereq. as they need to be bought together ... (but with promotional mod, i cannot add a formula)

-- Scenario 2

other scenario , if i buy the item 1 and 2 on eg order XXX