I will say that GH3 is easier - and they've clearly spent a lot less time making it
feel like you're playing along with the music.
As Yahtzee says in his wonderfully accurate review, however, the actual songs you have to play in Career Mode are arguably harder. There is a sudden destruction of the already shaky difficulty curve, as you're suddenly given practically impossible songs to play (at least, they are at first). As a seasoned GH2 player, I proceeded through the career pretty straightforwardly, but when I got to that Slayer song... I actually had to grease my fingers to play that damn song. true story.
What they've done, is abstracted the Guitar Hero III-playing techniques from the guitar-playing techniques. Like a lot of cheaply-done games it's more about "knowing the system" than having any real skill at the game.
It's just lucky that, as people have said, GH3 is a lot more forgiving than the previous ones.
Some might try to qualify the new "Devil went down to Georgia", but that hardly even qualifies as music to begin with
Don't you be knockin'. It is real music.