I have played Halo CE and 2 a gazillion times, but I had not played the Halo 3 campaign since a more than decade ago. It was ok but a lot worse than I remembered, playing it singleplayer has a lot of frustrating moments, like the end of Covenant when you have two beat two scarabs. Playing it coop this would be a blast, but alone it becomes a trial and error slog because you got to abandon your vehicle to destroy the first one, and then have to destroy the other scarab on foot.
And Cortana fucking sucked, easily the worst Halo level I've ever played, even worst than The Library on Halo 1.
In retrospect, now that I don't have the nostalgia goggles anymore, Halo 3 has probably the worst campaign of all the Bungie Halo games (I haven't played any of the 343 ones), even if it's still a decent game. The best levels are The Ark and The Storm. The best campaign is probably Reach or ODST. Halo 1 would be the peak if the campaign didn't jump off a cliff in The Library and the rest of the game was not backtracking with shittier enemies, but the run of levels between Pillar of Autumn and 343 Guilty Spark are probably the best in the series, specially The Silent Cartographer.