The Guitar Hero III: Legends of Rock gameplay remains unchanged from previous titles in the series; the player must hit notes as they scroll towards the bottom of the screen in time with music in order to maintain their performance and to score points. Hitting notes using the guitar controller requires strumming the strum bar while holding down the correct fret button(s). Playing with the standard controller only requires the correct button to be pressed. Notes can be sustained (in which the fret button(s) must remain held until the note is complete) and can also form a chord (two or three notes played together; however the song "Go That Far" by the Bret Michaels Band features four note chords on the rhythm guitar track on expert difficulty. It is the only song that features four note chords in the Guitar Hero series). The game also simulates hammer-ons and pull-offs for sections with a rapid series of notes. Missing a note will cause a performance meter to drop; if this meter drops too low, the player will fail the song, represented in-game by the band being booed off the stage. A string of 10 consecutive correct notes earns a multiplier to increase the player's score (up until four times), and special sections, marked by starred notes, can be used to build Star Power; when released (by tilting the guitar controller up vertically or hitting a button on a standard controller), the player's multiplier is doubled, the performance meter increases faster when a correct note is hit, and there is less of a penalty for missing notes. As with the previous games in the series, Guitar Hero III contains four difficulty levels: Easy (which uses three of the fret buttons); Medium (which makes use of four of the fret buttons); Hard (which utilizes all five fret buttons); and Expert (which adds no new fret buttons, but increases the number of notes and the general difficulty).
Two players using the same console can participate in Co-op Career Mode, which is arranged similarly to the single player Career mode, with one player on lead guitar and the other on bass or rhythm guitar. There are only 6 tiers of songs, with each encore song being unique to the co-op mode. There are no boss battles in this mode.
Guitar Hero III: Legends of Rock introduced Battle Mode, a competitive mode between two players either locally or over network play. The mode puts a twist on the Pro Face-off mode introduced in Guitar Hero II. Two players compete against each other, trying to make his or her opponent fail or lose by successfully playing "Battle Power" sequences (which replaces Star Power) to earn attacks that can be used against the opponent. Players can store three attacks at a time. To activate these temporary attacks, the player tilts his or her guitar upward, presses the select button (PlayStation 2 and PlayStation 3 releases), back button (Xbox 360 release), or minus button (Wii release). If neither player fails the song, they will face each other in a Sudden Death segment, where all further "Battle Powers" attainable are Death Drain attacks, which inevitably makes the opponent fail by draining his or her Rock Meter