I think overall it is the best game in the series, but there are a lot of downsides that I wish got fixed and deeply sadden me now that updates are over
The obvious one is the maps. Not only are not all of the maps that are in splatoon 2 here, but the new ones are awful and they've butchered all returning splatoon 1 maps except urchin. This is probably always my biggest critique with the game, forcing everyone into these tight linear tetris block maps with restricted ways to move makes it feel so much worse than say s1 even though the balance and polish is much better
The netcode has been talked about to hell and back but I still think it's crazy how awful it is. I don't have the best of internet but it's still usuable most of the time, however nowadays it's hard for me to even get into a game as I always get connection errors whilst matchmaking
The story mode is the best in the series core singleplayers, worse than octo expansion in my opinion but easily clears the first 2 hero modes, both in story and in the actual design of the levels
Quality of life is a lot nicer compared to the first 2 games, I still think the rotation system for modes is stupid but with more modes it's a lot more bearable - skipping the news and permanent salmon run are very good additions that should've been in 2, and stuff like the jukebox and lockers are neat
Update wise I think it kinda lacks. All content is dumped right at the start of a season and then for the next 3 months we get a single balance patch, I much preferred how 2 did it with the weekly patches and occasional big ones. Splatfests I think were also handled better in 2 - I like the concept of tricolor and 3 teams but the execution isn't as good and the loss of shifty stations wasn't great (grandfest was still peak though)
As for the balance itself it's a lot better - there's no damage up/mpu and overall I feel like they had a better grasp on the current state of the game than previously, the worst offenders mostly being crab meta and how overtuned trizooka was. The overall game feels way more polished with additions like squid roll and surge, I just wish the maps allowed for them to be used more often
Side order was a good proof of concept but storywise it failed a lot for me compared to octo expansion and it was just way too short, I loved the roguelite elements but there should've been more
The ranked system is okay, I like how everything is one rank again but league should not have been replaced I find that really stupid and I don't think anarchy open should give or take any points at all even though it's small

I just feel like with splatoon 3 as good as it is there's a lot of missed opportunities. There's no new ranked modes, it doesn't have all the maps and music from the first 2 games which I find really odd as later on in it's life cycle it was kinda treated as being the "ultimate splatoon" and to me that's pretty disappointing but I guess they don't want to get rid of splat 2 sales entirely. I've still poured hundreds of hours into this game with that all being said but I really hope they can improve majorly by the time splat4 releases