The 41-year-old former chef was found guilty of the murders of Gabriel Kovari, Daniel Whitworth and Jack Taylor.
The Old Bailey heard how Port lured the men to his flat in Barking, east London, after meeting on online dating apps before secretly giving them GHB,
He was also found guilty of seven offences of administering a substance with intent and three sexual assaults. He had denied 29 charges in total.
The body of Slovak-born artist Gabriel Kovari, 22, was discovered close to a churchyard in Barking on 28 August, 2014. On 20 September, the body of Daniel Whitworth, 21, was found in the same spot. The body of forklift driver Jack Taylor, 25, was found in nearby Barking Abbey ruins almost a year later, on September 14 2015,
Jurors are yet to reach verdicts relating to other charges, including the alleged murder of another young man, Anthony Walgate, whose body was found on Cooke Street in Barking on 19 June 2014, Sky News reports.
The judge has reportedly sent the the jury of 10 women and two men back out to continue deliberating.
They have been given a majority direction on the remaining counts, with the judge saying he would and said he would accept a majority of at least 10 to two.
UPDATE: Port was found guilty of the murder of 23-year-old Anthony Walgate.