That is exactly what my calculation does.
Say that you had a cart with the columns:
Quantity: 2
Price: 2000
SecondDiscount: 1250
MultipleDiscount: 250
and you use the formula I gave of:
([Price] * [Quantity]) - (([Quantity]>1)?(([Quantity]-1)*[SecondDiscount]):0) - (([Quantity>0])?(([Quantity]-2)*[MultipleDiscount]):0)
I'll replace the numbers:
(2000 * 2) - ((2>1)?((2-1)*1250):0) - ((2>2)?((2-2)*250):0)
Then I'll combine a few:
(4000) - ((2>1)?(1250):0) - ((2>2)?(0):0)
the syntax ((x)?y:z) will return y if x is true and z if it is false, so above, ((2>1)?(1250):0) will return 1250 because it is true that 2>1. Using that I can further simplify the result to:
4000-1250 or 2750
Which is the expected result given your parameters.
What I did is I combined tha three line OR statement that you gave:
(([Quantity]=1)?([Price]) OR
(([Quantity]=2)?([Price])+750 OR
(([Quantity]>2)?([Price]+750)*(([Quantity]-2)*500))
and I used algebra and php to combine it into a single calculation.