Where to start?
Season 2 is unrelated to season 3. S2 simply sets up HoT. If you have played HoT, then you would need to start with episode 1 of season 3 for anything to make sense.
People still seem to do all the maps, so until the expansion, you should be fine to catch up
If you haven’t played HoT, the S2 is the place to start.
Starting later in the story will be OK. You miss some of the setup, but … it’s like starting watching a TV series with character development in the middle. Just … accept there is a bunch of history, go with it, and you should be fine.