There's a pre-installed group, called "Newbies".
Make the post-count, for that, "3".
In Admin>Permissions, click on "Modify", beside "Newbie".
On the new page, put a check in the boxes marked "Post new topics, but hide until approved" and "Post replies to topics, but hide until approved".
You'll need to create a group for them to move on to, when they've made their three posts, if you don't have one, already.