Style boutique 3 is both a downgrade and an upgrade from its predecessor. On the plus side, a vastly expanded variety of fashion and a 27000 max inventory limit is a huge improvement over the second game and makes the outfit designing part of the game much better. The hairstyles available have seen much improvement as well.
A lot of the other activities are dead and gone, unfortunately. There is no color collecting, you just buy colors from a store. There's no modelling, and there's no fashion shows.
Money management is more clunky in this game, with side activities like doing makeup, nails or hairstyles not paying money to the player and furniture/clothes/colors being very expensive. The dark skin tones don't look as good, and there are much fewer characters with medium or dark skin - a noticable change from the second game. The player character is more detached from the main story, and doesn't appear in cutscenes with other characters. I also preferred it when ingame time was synced with real time.

So it's a situation where the core game mechanic is much improved, but a lot of the frills were cut off. Still, I would tell anyone that I liked this game.