1- Yes
2- Heroic/Legendary depending on number of players, but I barely play single player anyway
3- I hate camping
I not a huge fan because it's a really old format with its item/super weapon/pickups reliance, weird movement (particularly floaty air jumps), crap game types, lack of mid-game joining/quitter issues and long drawn out battles...
The list goes on but there are many draconian components that need re-working for it to survive.
I like the floaty air jumps (or I did in Halo2) as they offer you the time to use your jump to take "shortcuts" across the map, the problem is Bungie have not made any effort to make use of the floaty jumps in Halo3 (there was much more use for them in halo2)
I'm not too fussed about mid-game joining/leaving I think the way it is currently prevents abuse of the leveling system and other irritating behavior. I'm not saying its perfect but I feel its the best option given the circumstances.
In terms of movement (and shield regeneration) I'd wish things were slightly faster
Gametypes are mostly good there are a few less fun ones (territories again I prefer the Halo 2 implementation) and sometimes they use the "Wrong maps" for certain game types. I also wish (something Kreeeee and Alrik have also mentioned in the past) that you could veto the game and map independently
I think that throughout the Halo series they have had a lot of good weapons (and iterations of weapons) sadly they are not all in the same game, As I have said in this thread and others I like the fact that each weapon more or less has a purpose and a use, and that their are super weapons. I dislike FPS games where you choose your weapon at the beginning of the game and thats all you have available.
However I now feel there are too many different weapons, and a few weapons that do too similar a thing in Halo3.
TBH I'd do away with most of the dual wield weapons (i.e make them single wield only) and simplify it down to :
Sword/Shotgun
AR
BR/carbine (halo 2 variants)
Splazer/Rockets (with vehicle lock on)
Sniper/Beam rifle (halo 2 variants)
PP (halo3 variant)
PR
Needler (halo3 variant)
Frag/Sticky/Spike grenades (if I had it my way the spike grenades would work similar to the grenades in GOW2)
All the other weapons could be left available for custom games
In terms of power ups I like the overshield and the Active Camo. I'm still not sure how I feel about the equipment tbh. I'm not sure I'd miss them if they weren't there, however many of the maps have been designed with their use in mind (particularly the bubble shield)