I want to ask question about posting group setup when I make sales promotion like 3+1(free).
Yes, no standard way to do it. The way Thomas suggest, could be used, but still would require customizations, unless you don't care that the "free" item has a different item no. The tricky part here (I had the same requirement once) is often to get NAV to calculate the correct cost price etc. regarding the free item.
You would be much better off with a ready to use add-on. I just did a quick search, and there are many. One of them could be:
(PS: I have personally not tried or seen this add-on, so please don't see this as my recommendation, it just has the features you asked for - and more).