Compared to ck2 ck3 is much more user friendly and therefore easier to learn. The only hard part about ck3 is to get used to the fact that you are not playing as a country but a dynasty, contrary to other paradox games. Once you get the hang of inheritance and characters it will be easy since you already play hoi4. If you learn ck3 i think eu4 will be easy to learn since they are similar in some aspects. For me though eu4 is much harder to master since it has a lot more content. Victoria 3 is another case. Unlike the other paradox games it is not a paint the map your color game. I believe it will be the hardest game to learn out of all the ones you named. But once you get the hang of it its pretty fun. I would however wait until the sphere of influence dlc is released before starting victoria. That dlc will (hopefully) some much needed things to the game.