LeBron James, Chris Bosh, Dwyane Wade -- We're a JAZZ Band Now!!!

Forget the Big Three ... LeBron James, Chris Bosh and Dwyane Wade are now a 1920s-era JAZZ TRIO -- or at least they played one Tuesday night ... at Bosh's blowout 28th birthday party in Miami.

It all went down at Miami's Adrienne Arsht Center -- James, Bosh and Wade took the stage and surprised the entire party with an impromptu jam sesh ... and we're told, they weren't that bad.

Other guests included ... Timbaland, Heat President Pat Riley, and the entire Heat squad. Grammy winner Chrisette Michele also performed.