Very basic question : test case if line ordered quantity > 2 then get same quantity of free item B

Hello,

I know this is a very basic question in eBS advanced pricing but we would like to be able to get the same quantioty as ordered of free item B if the quantityti of ordered item A is gretaer than 3 (for instance).

So it would be :

if customer orders quantity of 3 of item A he gets quantity of 3 of Item B free.

if customer orders quantityti of 5 of item A he gets quantity of 5 of item B free.

If customer orders quantity of 2 of item A he doesn't get anything free,

etc...

We just don't know how to do that.