Senior players John Butcher and Jarrad Redden spent last weekend in hospital with the illness but have both since been released.
The three latest cases - announced just hours after senior coach Ken Hinkley told Adelaide radio station 5AA the club had undergone a "hospital strength clean" - involve players from Port's SANFL Academy, not its AFL list.
The trio was hospitalised but have since returned home.
In a statement released on Wednesday evening, the club said it would work "aggressively to eradicate the virus from the club facility, if indeed it exists there, and ensure that the cycle of infection is halted before any staff or other players are affected".
Port's doctor Mark Fisher contacted the communicable disease branch of the South Australian Department of Health to work out the best approach to contain the outbreak.
He said both Butcher and Redden would be quarantined away from the club's Alberton base for up to another 10 days.
The club's facilities are partially shared with the local community, which has made the location of the initial source of the infection difficult to identify.
“What has occurred here at Port Adelaide is a typical community outbreak that is not unusual within a small community environment like a football club,” Dr Fisher said.
“This virus is particularly prevalent within the community at this time of the year and it is most likely that someone has brought it into the club and it has spread from there.
“The good news is that John Butcher and Jarrad Redden are both out of hospital and on the mend resting at home. We would expect them to remain away from the club for the next seven to 10 days.
“Our three Magpies players who were diagnosed in the last 48 hours have all progressed very well and are already home recuperating.”
Port CEO Keith Thomas said staff, players and players’ parents had all been briefed about the situation and that anyone displaying signs of illness had been instructed to see a doctor and remain at home.
The outbreak has led the club to "significantly increase" its cleaning protocol.
“Port Adelaide’s facility management procedures are already at AFL standard and although this virus is most likely to have been brought in from outside the club environment, it is clear that a fastidious commitment to our hygiene protocols is the best defence against the spread of infection,” he said.
Twitter: @AFL_Harry
