Product Discount Triggers Free Shipping

I want to sell a product “buy 2 get 3”, can't do that with promotions so I used product discounts. Can only do that by limiting the sale to 3 otherwise all amounts more than 3 get the discounted price. Now I got it but the cart is not charging shipping. I have a promotion of free shipping over $50 and the discounted 3units costs $39.90. I also have set the Shipping methods for free shipping over $50. Can anyone tell me what is going on here?

thanks

I think what is happening is that the discount acts like a cart discount and is deducted from the total. The total before the discount is over $50 and so triggers the free shipping. I found that I can accomplish this discount with the Cart Promotion but this results in free shipping. I tried to do this with the Catalog promotion but cant get the buy 2 get 3 result. Is there some other way to do this?

thanks