Philip Carlyle, 47, was shot four times in the head in his office on Queensland's Gold Coast in April 1997.
Police have long suspected that the father-of-three knew his killer because there were no signs of a break-in, but progress in the case had been elusive.
A fresh cold case investigation began last year.
On Tuesday, detectives charged a 68-year-old Gold Coast man with murder.
They also charged a 69-year-old woman with making false statements to police, and a 66-year-old man with being an accessory after the fact of murder and having an unsecured weapon.
Police rewards totalling A$400,000 (Â£240,000; $300,000) had failed to provide progress in the case.
However, local media reported that detectives last year discovered a fresh clue relating to an uncommon type of ammunition used in the crime.
In 2014, police identified the ammunition as Swedish-manufactured Norma brand .32 ACP 77 grain.
Earlier this year, lead investigator Detective Sergeant Rod Seaman told the Australian Broadcasting Corp that police were closing in on a suspect.
"We believe that we have identified some ammunition which has come from that same batch which we have seized from a person of interest," he said.
Mr Carlyle, a marketing manager at a computer business, was described by relatives as a generous and loving family man.
Australian police have arrested three people in what could mark a breakthrough in a 20-year-old execution-style murder case.