Technically speaking, yes.
However realistically for most students, it's not doable without the guidance of a teacher who understands Harder 3U very well.
There quite a bit of extra theory you need to learn that again technically you can deduce from 3U knowledge purely, but it's not easy to do so yourself.
Classic example is Strong Induction. In 3U, you learn three types. Series, Divisibility and Inequality problems. In Harder 3U, the idea of Strong Induction is introduced.