The case has been one of Australia's highest-profile mysteries for more than two decades, and came to be known as the Claremont serial killings.
Bradley Robert Edwards, 51, denied killing Jane Rimmer, 23, Ciara Glennon, 27, and Sarah Spiers, 18, in Perth.
On Thursday, a judge convicted him of Ms Rimmer and Ms Glennon's deaths.
But Justice Stephen Hall said there was not enough evidence to convict him of murdering Ms Spiers.
The judge-only trial was Western Australia's longest-running and most expensive murder trial, taking place over seven months and involving over 200 witnesses.
"Every Western Australian is thinking of the families of Sarah, Jane and Ciara today," state Premier Mark McGowan said after the verdict.
"What happened to these young women changed our state."
The women disappeared on separate nights in 1996 and 1997, from a popular nightlight spot in Claremont, a Perth suburb.
In the case of Ms Rimmer and Ms Glennon, Justice Hall said Edwards took them from the street, drove them to the city's outskirts and used a knife or other sharp object to attack them.
In both cases the victims had defensive wounds, showing they tried to fight back against the telephone technician. Both were fatally wounded in the neck before Edwards hid their bodies in the bush, the court heard.
They were discovered a few weeks after their disappearances. But Ms Spiers, the first to go missing in 1996, was never found.
It remains unclear how Edwards trapped the women. Prosecutors said he either abducted them or lured them into his work car, which looked like a taxi.