Season 3 was not just another set of bosses. NC added the Hall of Illusion Expedition, new Nightmare bosses and several changes intended to make character growth less repetitive. The update also altered some seasonal ranking criteria so comparisons increasingly reflect class rather than a single pooled leaderboard.
Where to begin if you are behind
Before buying a catch-up item, inspect what the current progression systems already offer. NC described a Warm Welcome boosting pass and Shugo's Growth Support Ticket in the Season 3 announcement. Their exact availability and contents can depend on date and region, so do not plan around an old promotion without checking in game.
Hall of Illusion was introduced with an item-level entry threshold and materials tied to Daevanion Yustiel. That makes it a specific progression target, not a universal answer for every character. Use NC's Season 3 announcement as the factual starting point; use your current character sheet to decide whether the run is worth your time.