
The two-year release gap between Rock Band 2 and Rock Band 3 is fascinating, as it showcases two different environments where rhythm games existed. The first was at the height of its power, the pinnacle of rhythm-based gaming through full band gameplay, challenging setlists and songlists, and a robust library of songs that allowed for tons of possibilities. Rock Band 2 was a game released at the right place, at the right time, to take advantage of a white hot fad.
That fad imploded by the time Rock Band 3 came out. Within two years, fatigue for rhythm games was on the rise, rival Guitar Hero was going dormant, and Harmonix's publisher, MTV Games, was concerned about the massive drop off. By 2010, the genre saw a 50% drop in revenues from the previous year. Timing didn't help, as the world was in a global recession that started in late 2008, but the over-saturation of rhythm games is always a major contributing factor. Considering that only four major rhythm games were released in 2010, compared to upwards of a dozen in 2009, it is hard to reconcile the fact that the genre self-imploded.
Harmonix was well aware of this, and felt that they could try to re-energize the rhythm genre by advancing it to the next step, the pie in the sky ideal of marrying the gameplay aspects with the music industry. All of this to say, Harmonix wanted to innovate with Rock Band 3, instead of resting upon their laurels.

To that end, two major additions to Rock Band 3 were heavily promoted as the game was released. The first was the inclusion of a new instrument, the keyboard. Keyboard functionality was purposefully designed to answer for any staleness in gameplay. It also allowed MTV games a new peripheral, the keytar controller, as a purchasable option for Rock Band 3.
The keyboard worked as you would expect, players would strike specific keys on the display of the game to score points. Each of the keys are color-coded by the board, with certain note sections a certain represented color in game. In the basic gameplay mode, the five white keys from the middle C to G range correspond to the notes on screen, and would be played as you would normally do with a Guitar.
The game is mostly centered around the keytar controller, with a lot of the games setlist being keyboard or piano heavy to provide a justification for playing the game's songs. This does make the setlist unique for Rock Band 3, but it is also arguably the weakest on-disk setlist for the series. Some of the songs didn't include keyboard support either, with 20 of the game's 83 songs not utilizing the new instrument. Ironically some of those songs are nice additions to the Rock Band ecosystem, though a few are also repeats from Guitar Hero, like "The Beast and the Harlot" by Avenged Sevenfold, "Crazy Train" by Ozzy Osbourne, and "I Wanna be Sedated" by The Ramones.

Songs that do include the keytar however include some really good pulls. David Bowie's "Space Oddity" and Dio's "Rainbow in the Dark" are great selections, and we get solid hits from Chicago, Tokio Hotel, James Brown, The Doors, and more. Perhaps the best song to use the keyboard would be Queen's "Bohemian Rhapsody", which combines a bit of each instrument with the muscular lyrics of Freddie Mercury to make one of the best songs to play with a full band.
This is the weird problem with the setlist of Rock Band 3. It's very good and offers a lot of variety, but the focus is definitely less band centric due to the keyboard. Exceptions like "Bohemian Rhapsody" help it a lot, but other songs are heavy keyboard-focus. The Doors "Break on Through (To the Other Side)" offers little for guitar, bass or drums, same with Amy Winehouse's "Rehab" or Warren Zevon's "Werewolves of London." This is a criticism insomuch as the focus on the keyboard does make other instruments seem like they are in the periphery of the games scope.
Harmonix likely realized this too, as their second big improvement to the game would be Pro Mode. A first for the Rock Band series, Pro Mode was designed to cater to the player's individual instrument to learn how to play the instrument in real life, helping to play the songs note for note either offline or online. Pro mode comes with its own difficulty modifiers, so easy mode, for example, would reduce the number of notes to hit, but requires players to match the proper fingering for guitars, or hitting the correct cymbals for drums. The difficulty ramps up by introducing other aspects like chords in medium pro mode, to full on chord pulls and multi-note progression by the time you hit expert pro mode.

This does try to address the lamentations of game reviewers at the time, who argued that games like Rock Band should help in training players to play guitar. After the abject failure of Power Gig, there was still a hunger for professional playing. To that end, Harmonix revamped their entire tutorial modes for pro-mode, with trainers developed by the Berklee College of Music to help players learn how to play their instrument. These new trainer modes use music created by Harmonix to help ease players into more realistic playing.
The tools for pro mode are fantastic as a teaching tool, and really set up Rock Band 3 as the go-to game for learning how to play an instrument. Each instrument is also designed specifically for this. MTV games would release "pro" versions of each instrument, except for the keytar, which already had it's pro-version features baked into the controller. It would take a lot of time to go through each controller. Just to get a clue of this, two pro-controllers for guitar would be released, the Fender Mustang Pro-Guitar would be a MIDI 102-button/string controller, while the Fender Squier Stratocaster 6-string guitar and MIDI controller would basically be a real-life guitar peripheral that doubles as a gameplay controller. Both contained new fret sensing abilities to provide instant feedback on where the players fingers would be on the fretboard, and both can be used outside of the game as real electric guitars.
It is honestly impressive that MTV Games and Harmonix were able to pull off pro mode. It could have easily been just a gimmick, similar to what Power Gig claimed it can do, but Rock Band 3 actually pulls it off and provides a secondary gameplay mode that could take expert players time to master all over again. It is also a mode that compliments the gameplay of Rock Band 3, as you can seamlessly include pro-players with "basic" players who are used to the more traditional Rock Band gameplay, so there is no segregation between groups.

As impressive and unique pro mode is, this did come with a cost. Outside of the heavy cost for instrument peripherals, Rock Band 3 also ended the streak of all DLC from the Rock Band network being playable between games. By the release of Rock Band 3, there were close to 2,000 different songs available between the games. Rock Band 3 would allow transfer of most songs from Rock Band 1 and 2, Lego Rock Band, and Green Day: Rock Band, but new DLC for anything prior to the release of Rock Band 3 ceased. The reason was simple, the inclusion of keyboards and pro mode forced a restructure of the Rock Band Network.
I should mention the Rock Band Network. Launched in 2010 for Rock Band 2, the Network allowed bands and other labels to release their own mixed songs for purchase by players. Harmonix and MTV Games gave bands the ability to chart their songs using the same tech they do, and go through an authorization process where tons of indie bands, albums, and songs would get released. Separate from the regular DLC releases from Harmonix, the Rock Band Network would balloon song selections heavily, providing tons of choices for players up until 2011, before transferring over to the 2.0 version exclusive for Rock Band 3.
The cost of making pro-tracked songs and the use of keyboards also increased the cost of DLC, Harmonix often offered songs in both basic and pro mode, with the additional cost being about $1-$2 extra per song. Pro-keyboard, harmony vocals, and pro-drums would be new features added, though guitar and bass pro charts were too complex for the Rock Band Network to release. The loss of DLC for Rock Band and Rock Band 2 was devastating, but Rock Band 3 would continue to get support until 2013.

Many critics say Rock Band 3 is the best game in the series, and like all of the Rock Band games, it is definitely in the running in some form. For me though, it is a game that I respect more than I enjoy, simply because the innovations in Rock Band 3 are great, but not what I personally enjoy in rhythm games. The term 'feature creep' comes to mind; a lot of features, gameplay tweaks, and additions make an overall giant package, but it feels a bit bloated because of them all. The keyboard focus and pro mode focus were bold and necessary moves in 2010 though, but it doesn't right the ship overall.
Rock Band 3 would be the final game from MTV Games and the final game published by Electronic Arts. Sales were good, but like other rhythm games, the overall sales were lower than expected, leading to a dormant period before Guitar Hero and Rock Band got one final tour in the mid 2010's. Rock Band has been the more consistent series between the two, but Rock Band 3 is definitely a major turning point of gameplay expectations; there really is nowhere else to go with the series, and perhaps Harmonix sensed that itself, waiting until 2015 to release their final game in the series, Rock Band 4.