opencart category quantity discount - php

Question

I want to set discount is as follows.
Suppose, My one product is for £4.29.
If buyer buys 3 or in multiple of 3 like 3,54,39 or 189 etc, will cost him as £3.33/product.
If buyer buys bulk order other than multiple of three like 4 or 14 or 2 or 38 etc, than it will cost him as regular price of £4.29/product.
* One product will cost £3.33 only and only if bulk order is made with multiple quantity of 3. *
I found some setting in the discount section of each product. It works fine as per my requirement. (reffer attached image)
![enter image description here][1]
But it's hard to mention it for each multiple of 3. For Eg. What if a customer wants to 300 products. For that I need to write till the 300.
Is there any extension available for this or any one have other solution for this problem.
Thank you for your answer.

Answer 1

If am getting you right, you are able to calculate discount for all other quantities except for those in multiples of 3.
If that is the case, my suggestion is:
qty;
if ((qty % 3) == 0) {
// multiply qty by 3.33
price = qty * 3.33;
}
else {
// use your existing (working) formula
}
I hope this works.

Answer 2

twist added into the problem.
There are three categories with same price. Assume that its £4.29.
If a customer buys any of these category product with a total quantity of 3 or multiple of 3 than one product price will be £3.33
If a customer buys 10 products than 9 products will cost him as £3.33/product and remaining 1 product will cost £4.29.
Customer is allowed to buy 3 diffrent products of three different categories or he may buy 2 from one category or 1 from another category.
I hope you guys will understand the problem and give the bestest solution possible.