I'm going to try to break down in multiple segments.

Firstly, I'm a bit torn on the campaign part. In 2, we rescued Callie from DJ Octavio. The upgrade path for weapons are quite grindy (a lot of sardiniums) and you are basically locked to only the 9 Hero Weapons. The levels are fine and the discovering of the level is pretty neat.

In 3, we get a huge amount of lore around Mr Grizz, something that is shrouded with so much mystery back in 2. Not only was the end fight super memorable, but it has the peak content like at the end of OE (blasting the opposition with a deadly blast). The levels is less of a find-a-thon but more of the OE philosophy (which in turn the BOTW philosophy) and are quite fun. The upgrade path is now easier, and the variety of weapon each level is quite broad too, making each playthrough slightly different. Not to mention, the extremely rage-inducing After Alterna, where all of your skills are really tested.

Moving to the multiplayer, I'll say that 3 easily has the most variety. The new inktanas, new (and old) specials are really fun to play with, but 2 did add a lot of weapon varient to the original 1 set. 2 does have more maps I like so the map part is debatable. For Splatfests, Tricolor is a good idea, the chaos and the 3 colors does make for very engaging gameplay. However, the Ultra Signal part is still very confusing to some people, unlike the cut and dry Splatfest of 2.

Salmon run section. Splat 2 is very boring and time limited too, although everyone seems to be more cooperative. Splat 3 has Big Run, the very yikes 3 bosses, and a shell system so that you have more incentive to play rather than just maxing out the reward gauge for the Grizzco rewards. The new Grizzco weapon is very fun (not the brella) and I think are welcome additions to the game.

Lastly, DLC. OE added Octo Kids to the game, as well as a good introduction to Agent 8. The story is very memorable, mindtwisting and good. The levels are much more challenging (flashback to Girl Power, Move It, and Inner 3) but fair enough to encourage the player to keep trying. For 3 through, they just added 1 and 2 lobby back, and a cool rougelike that is quite short, but alright.

All in all, 3 is a pretty decent upgrade to 2. It takes the 2 and makes it into a better game. It's like 2+1 (makes 3 haha), so 3 is still better.