District Attorney Neal Pinkston said Bianca Horton, who had been set to testify in a murder case, was tortured before being shot multiple times.
The state is seeking the death penalty against Courtney High, Charles Shelton and Andre Grier in the 2016 slaying.
DA Pinkston said the killing was "heinous, atrocious and cruel."
Ms. Horton had been set to testify against Cortez Sims in a Jan. 17, 2015, mass shooting at College Hill Courts. Ms. Horton was grazed by a bullet and her young daughter, Zoe, was left paralyzed. Talitha Bowman was killed. Marcel "Baby Watts" Christopher was shot, but survived.
Sims was found guilty of murder, though Ms. Sims was not there to testify and Christopher refused to do so.
High, Shelton and Grier are among 55 defendants the state says are members of the Athens Park Bloods. They are being prosecuted under the Racketeer Influenced and Corruption Organizations Act (RICO).
Courtney High was in court before Judge Tom Greenholtz on Monday, and he pleaded not guilty to 11 counts, including first degree murder and aggravated assault in the Horton death.