The former security guard was convicted last year of three counts of first-degree murder, and special circumstances in the deaths of 15-year-old Yvonne Coleman, 36-year-old Verna Williams and 32-year-old Deborah Jackson, also known as Harriet McKinley.
He then left his victim's naked and posed after death.
Authorities say DNA evidence linked Hughes to the killings, which occurred between 1986 and 1993.
Hughes is already serving a life sentence for killing four women in the early 1990s.
The killings were investigated by the Los Angeles Police Departments Robbery-Homicide Division, which was examining unsolved murders of as many as 90 women in South Los Angeles between 1960 and 1997.
Police later concluded at least three serial killers, including Hughes, were stalking women during a crack cocaine epidemic that led some women onto the streets as prostitutes in order to support their habits.
Pizza deliveryman Chester Turner was convicted in 2007 and sentenced to death for killing 10 women and a fetus in Los Angeles in the 1980s and '90s. He has been charged with four additional killings and ordered to stand trial.
In addition, investigators arrested Lonnie Franklin Jr., a South Los Angeles mechanic, last year.
He has pleaded not guilty to 10 counts of murder and one count of attempted murder.
Prosecutors suspect Franklin shot or strangled women and girls between 1985 and 2007, dumping their bodies in alleys near his home.