The 20-year term was a minimum mandatory sentence that was required because MacKay was convicted of causing the death of patient David Wirick, who died after taking two prescription drugs.
But since that conviction, the U.S. Supreme Court in a similar case held that controlled substances like the oxycodone and hydrocodone Mackay prescribed to Wirick must individually be capable of causing a death for a mandatory minimum sentence to be enacted.
Prosecution experts testified at the trial that the death likely came from a combination of drugs, while the defense asserted that pneumonia could have killed him.
In May, Benson shaved 17 years from the punishment after the 10th Circuit Court of Appeals sent the case back to him asking for a more specific finding on how much prison time the doctor was given for each charge.
The judge said then that a sentence of three years was appropriate to reflect the seriousness of his conviction and to act as a deterrent to others. Benson also cited MacKay's nearly two years in prison where he has been a "model inmate" who completed 22 educational courses and acted as a tutor to others.
In addition, the 66-year-old MacKay suffers from diabetes, coronary artery disease, hypertension, high cholesterol, obesity, gout and cirrhosis of the liver and has had numerous surgeries, Benson said.
Prosecutors asked Benson to reconsider the new sentence, saying the victims had been denied the right to be heard and that the judge had ignored some of the evidence including the fact that a death occurred in deciding on a sentence.
"This man was a drug dealer," Assistant U.S. Attorney Michael Kennedy said Tuesday, adding that the only difference between MacKay and a street dealer was "one has a medical license."
Benson responded he had considered all of the evidence and the arguments in the case presented at trial and in post verdict proceedings. He described a three-year term for a first offender as "a serious sentence" and said he was not forgetting Wirick in meting out a punishment.
Benson also noted that the victims had spoken at previous sentencing hearings, but he allowed Wirick's widow and daughter to make statements again Tuesday.
Both women described Wirick a Navy veteran who had worked as a firefighter, paramedic and engineer as a gentle, intelligent man who loved his family and the outdoors. A back injury he suffered from an on-the-job accident while he was a firefighter and subsequent surgeries led to a reliance on pain medication, they said.
Susan Wirick tearfully told Benson that her husband had crossed the line between pain management and drug addiction, but MacKay still kept on prescribing medication, despite her asking him to stop. She believes the doctor did it for material gain.
"I am angry," Susan Wirick said. "Three years is all he's going to get?"
MacKay is incarcerated at the low-security Terminal Island federal prison in San Pedro, Calif., according to the U.S. Bureau of Prisons. With time off for good behavior, he could be released before the end of the year, according to Peter Stirba, his defense attorney.
MacKay was indicted in August 2010 on 129 charges related to prescribing more than 1.9 million hydrocodone pills and nearly 1.6 million oxycodone pills between June 1, 2005, and Oct. 30, 2009, and accused of pushing 80 to 120 patients through his office in an eight-hour workday.