The 50-year-old former Telstra technician is fighting charges he murdered secretary Sarah Spiers, 18, childcare worker Jane Rimmer, 23, and lawyer Ciara Glennon, 27, in 1996 and 1997 after abducting or luring them into his work car when they were intoxicated and alone.

In a brief opening address on Tuesday, defence counsel Paul Yovich urged Justice Stephen Hall to "beware the tendency to smooth out the rough edges" in the prosecution's case.

"The defence is simple - it wasn't him," Mr Yovich told the Western Australia Supreme Court.

"We are not pointing the finger at any specific person, all we are saying is the nice, neat picture the state wants to present ... is not the full picture.

"The proper approach in any case is to fit the case theory to the evidence, not to try to fit the evidence to the case theory."

Mr Yovich said the DNA and fibre evidence could have been contaminated, adding storing of materials in the 1990s was much less sophisticated.

Some DNA samples in the case, when tested, were shown to match PathWest scientists.

Mr Yovich said some of Ms Glennon's fingernail scrapings were actually clippings by a mortuary technician and had previously been assessed by scientists as "debris only".

Edwards did not have an explanation for how his DNA got under Ms Glennon's nails, but accepted a "direct scratching event" was more likely than chance social contact.

Earlier in the day, prosecutor Carmel Barbagallo said the investigation started to gain momentum in May 2013, when cold case detectives boxed a silk kimono dropped during an attack on a sleeping 18-year-old woman in her Huntingdale home in February 1988.

The kimono was finally tested in November 2016 and semen stains allegedly matched swabs taken from a 17-year-old girl who was twice raped at Karrakatta cemetery in 1995, and DNA found under Ms Glennon's fingernails, some of which broke off as she fought her attacker.

Investigators looked further into other offences committed in Huntingdale, where Edwards grew up.

A breakthrough came on December 16, 2016 when fingerprints taken from an attempted break-in in the same suburb in 1988 matched Edwards, who was in the national database after a 1990 attack at Hollywood Hospital.

Detectives swooped four days later, testing a Sprite bottle he discarded at a cinema where he watched a movie with his stepdaughter.

It was another match.

Police then arrested Edwards at his Kewdale home.

Detectives interviewed him for about six-and-a-half hours, during which he claimed he was only "a little bit" familiar with Claremont and had no association with the area so had no reason to go there.

Bank records show that was a lie as he did not want to implicate himself, Ms Barabagallo said.

He also "feigned disbelief" his DNA was on the cemetery victim.

Ms Barbagallo said fibre evidence found on Ms Glennon, Ms Rimmer and the rape victim was also a critical part of the case, holding "a lot of discriminating information" and providing another forensic link to Edwards.

Justice Hall, who is presiding over the trial without a jury, asked Ms Barbagallo if she would argue the murders were sexually motivated as she has submitted for the Huntingdale attack.

"We're not necessarily nailing our colours to the wall on that," the prosecutor replied.

"Certainly, Ms Rimmer was found naked."

Edwards confessed to the cemetery and Huntingdale attacks last month.

Australian Associated Press