Like with most shows they only had so much creative ideas going into this and blew all their load into season 1.
Season 2 was boring while season 3 i kind of enjoyed but clearly they are starting to scrape the bottom of the barrel in terms of characters, drama and just basic plot.
Moreover instead of doubling down on what worked, they introduced way too many characters without any actual charisma and diluted the whole thing. I'm also kind of surprised that they totally blue-balled Steve and made Robin lesbian for no good narrative reason other than inclusion? I don't hate it but it feels calculated and the whole thing at this point is basically in need of some fresh ideas. It is held up by good actor chemistry, well-written dialogue and good overall pacing so it's still a show I'd watch, it's just not exceptional anymore and risks becoming simply derivative if season 4 doesn't shake things up massively.