I've put about 20 hours into HoI3 and I still make "rookie" mistakes like having too many units in an area and getting a stacking penalty or sending my tanks into a mountain. :-/
I went into CK2 from EU3 and was able to learn and master the different systems quickly. If you want a good place to begin in CK2 I would recommend one of the Irish dukes. You can unify Ireland without outside interference and go on to conquer Scotland/England or wherever you choose.
If you really enjoy the organization of military corps and armies, then HoI3 might be for you. There is just so much more detail in the controlling of the military compared to CK2, Vicky2, or EU3 that I felt lost for quite a while.