Promotion Item : Buy 2 for 8SGD

Hi,

Yes. You need to configure three prices in your product like that:

price          min quantity
5                1
4                2
4.33333     3

Hi,

Well, alternatively, you can also configure autoloaded coupons with "minimum quantity" of 2, 4, 6, etc and the amount you want to discount. Then, during the checkout, the appropriate coupon will be loaded automatically based on the number of items of that product.
That requires either the Essential or the Business edition.
It's also possible to write a small plugin if you want to automate that and override the price of the products or add fees (that can be negative) dynamically, on the cart. So with that, you can do any kind of pricing plan you want, but it requires some PHP knowledge.