Prosecutors said Port invited the men, who all were in their 20s, to his home, where he gave them drinks containing deadly doses of GHB so he could rape them while they were unconscious.
Police initially did not link the deaths, which took place over more than a year.
A jury at the Old Bailey this morning unanimously found him guilty of the murders of Gabriel Kovari, 22, Daniel Whitworth, 21, and Jack Taylor, 25.
The jury has yet to reach a verdict on other charges, including the alleged murder of fashion student Anthony Walgate, 23, from Barnet on 19 June 2014.
Get breaking news right in your inbox..
The Old Bailey court also heard testimony on how Port embarked on an elaborate cover-up of the murders, disposing of victims' mobile phones and repeatedly lying to police.
A man has been found guilty of the murder of Daniel Whitworth from Gravesend, along with three other men he meet on a dating app.
It comes as seventeen police officers face investigation for possible misconduct over the catalogue of failures in catching the killer.
He insisted that work was being done to improve officers' understanding of so-called "chemsex" - when gay men take drugs before having sex - and to make sure frontline bobbies could spot when a death might be suspicious.
He stopped short of actually apologising to the families of Port's victims, but instead said that he would be writing to them to "express our honest condolences", and he has "offered to meet them if they would like to do so, both now and at the conclusion of the IPCC investigation".
The court heard Port had an insatiable desire for boyish-looking men he referred to as Twinks.
The Independent Police Complaints Commission (IPCC) is investigating whether homophobia played a part in the errors.
Port denied all the charges against him but was found guilty of the murders of the three men as well as a range of sexual offences against more men.
"From what we've seen as an investigation team, this is a highly devious, manipulative and self-obsessed individual".
Police have not ruled out the possibility there may be more victims of Port's sexual abuse and appealed for them to come forward.
The second man to die was 22-year-old Slovakian Mr Kovari, who was staying on Port's sofa as a temporary flatmate.