Trouble is, it isn't really guitar hero anymore if it's more than a guitar. Looks like a good attempt to freshen up a series that's going the way of Tony Hawk and being the same every year.
The previous games before Guitar Hero 3 aren't exactly boring (for me, at least), so I didn't think there was anything wrong with the series. As a matter of fact, old Tony Hawk games sell for rather cheaply, but Guitar Hero was released in 2005 and that is still popular among second-hand buyers. Also, Guitar Hero 3, like Tony Hawk games, was developed by Neversoft. It's not that the game is losing interest, but the reason the drums and mic have been added is to compete with Rock Band, which has been out for a similar time as GH3 in the US.
Even if the games did seem samey, they're not exactly bad: it's just a different set of songs in each game.