That `butlast` removes the last three elements, but here you were supposed to remove the first three elements. So I guess `cdddr` is what you need.

Also, the way the article uses `remove-if-not` is all wrong. The `:start` argument to `remove-if-not` doesn't mean it starts taking elements after the third one, rather it means it starts filtering after it. So

  (remove-if-not #'evenp '(1 2 3 4 5 6) :start 3)

doesn't produce (4 6) as the article assumes. It produces (1 2 3 4 6).

And the `:count` argument doesn't mean it returns 5 elements, rather it means it removes 5 elements. So

  (remove-if-not #'evenp '(1 2 3 4 5 6) :count 1)

returns (2 3 4 5 6).