There's infinite reasons why you or anyone may be finding S3 more difficult. There's a larger player base, which widens the skill level range/increases the number of players within any given range. It's easy to blame stuff like the maps being bad (which are admittedly questionable in design and can contribute to matches feeling unbalanced/un-fun) or the matchmaking broken (but let's be honest, the matchmaking in S2 had issues as well), and some of these criticisms have merit.
I'm not sure how long you've played/what rank you are, but since you mentioned having an easier time earlier on in S2, it's possible you had a much higher mechanical skill than players who were also lower ranked, so you could win despite making mistakes in positioning/decision-making, but now that you're in higher ranks, that skill gap is gone. Everyone hits a wall at some point, so maybe it's just your time. You can even think of facing harder opponents that you can't beat as a good thing; it means you've come a long way and that you still have more to learn if you want to.
On that note, I can't give tips since I haven't seen your gameplay, and I don't know what issues you might be having there, but I think that one thing you're having trouble with is your mindset. Losing is discouraging/frustrating, but it's important to not focus too much on the results of any given match. Being worried about results leads to not focusing on your current gameplay and learning in general, which leads to losses, losing leads to a worse mindset, a worse mindset leads to even worse play, and so it repeats and just becomes un-fun. You're losing and you're questioning if you're "good" at the game, but what even is "good," really?
You say that you feel like you "should" be winning ("even with all my experience from its predecessor, i can't seem to win anything in splatoon 3"). But why is that? Saying that you "should" do something adds so much pressure and stress, and it makes you feel worse if you don't live up to expectations that don't have to be there. Say, if you've played 500 hours, and you 1v1 with someone who has played 200 hours, do you think that you "should" win and would it feel worse if you did lose? Some new players learn faster. Some new players have prior experience with shooters/multiplayer games in general. Some players play thousands of hours, but they're still at a lower skill level because they just want to play casually and not worry about improving because that's what they find most fun.
Changing your mindset is easier said than done, of course! I don't want this to come across as, "Just be positive!" Because, yeah, that's not helpful and actually goes against what I'm saying. It's more so that this is something within your control that can be worked on in measurable ways instead of just saying you're "good" or you're "bad," even if the maps are bad, even if the matchmaking is trash, even if you're not looking to be tryhard at the game and work on improving (and that's totally fine! If you want to play casually and maybe just work on not focusing on the results and having fun regardless, that's great too). A good rule of thumb is if you lose 3 matches in a row (i.e. a Series goes completely wrong), you might want to take a short break. Or if you're really feeling frustrated, take a longer break from the game. Or even just play a different mode, or try some new weapons in TW, or play with friends instead of randos.
If you're wanting to try and proactively improve at the game, have you tried reviewing your replays? Replays are great for seeing what things you did well and can continue to do and what things you can work on improving. They're also helpful because it's easy to fall into the trap of blaming your teammates or thinking that your opponents got a cheap splat against you, and seeing your play from the other players' perspectives can contextualize things.
In battle, it's easy to say, "Ugh, my teammate rushed in and died. It's their fault we lost," or, "How did that E-Liter hit that shot?! They can see me from that angle?!" but watching replays with a calm mind can change that to, "Actually, I rushed in earlier and left my teammate in a bad spot where time was running out, and they didn't have much of a choice except to do the same. I should be more patient and wait for my teammate first," or, "Actually, that snipe was super sick. That was a difficult shot, and that E-Liter nailed it." Complimenting the other players and also laughing at your mistakes instead of worrying about being "bad" just helps in general with keeping a positive mindset.
Asking others to review replays is also great! I'd offer to help with that, but I'm pretty new to the game myself (only started playing 2 earlier this year), so I'm not sure that I'd be the best person to ask.
