There are multiple artists with this monikor:

1) G3, an almost annual live music show and concert tour featuring three select guitarists of the generation; hence its name.

Organized by guitarist Joe Satriani in 1995, G3 has toured most years and has featured many guitarists since its inception, including:

Steve Vai, Yngwie Malmsteen, John Petrucci, Robert Fripp, Eric Johnson, Kenny Wayne Shepherd, Uli Jon Roth,
Michael Schenker, Adrian Legg, Paul Gilbert.

and many other special guests, such as Steve Lukather, Steve Morse and drummer Mike Portnoy.

There are three official live releases of G3:

Live in Concert (1996)
Live in Denver (2003)
Live in Tokyo (2005)

For more information, visit the G3 section at Joe Satriani's official homepage.

2) G3, a Peruvian hardcore punk band.

3) G3, a Christian/Gospel group. Read more on Last.fm. User-contributed text is available under the Creative Commons By-SA License; additional terms may apply.