Cart Rules Help Req - Buy X for £Y

[fastfuse]

Hopefully someone can help with a cart rule issue we are having.

 

We have a promotion of buy 3 items for £9 (Usually £3.50 each). This works fine, but if someone buys 6, they still only get the single discount.

 

Essentially we would like 3 for £9, 6 for £18, 9 for £27 etc etc...

 

Is there a way to set the cart rule so it automatically multiplies up like this ?

 

The only other way I can see it working is if we make a cart rule for 3, 6, 9, etc... but that's far from ideal because some people may buy over 50, and expect the discount.

 

Please help :-)