Newsroom reports the inquiry began after staff members "raised serious concerns" about Ross' behaviour and conduct over the past year.
The inquiry is being carried out on behalf of Parliamentary Service, the employer of the two women and one man.
It is not clear who is carrying out the inquiry, but Newsroom reports it has been told an "outside organisation" would be looking into the matter.
In a statement to the Herald, a spokesman for Parliamentary Service said it "won't be providing comment on this particular matter".
October 2018, just days after Ross' dramatic falling out with the National Party in which he publicly called leader Simon Bridges a "corrupt politician" - which Bridges denies - Newsroom published the accounts of four women who accused Ross of harassment.
Two of those cases, according to Newsroom, involved women Ross was having a sexual relationship with.
Three of the women said they had breakdowns, while one of them said she sought medical help.
The allegations prompted Speaker Trevor Mallard to launch an inquiry into the culture and conduct of Parliament.
That inquiry laid bare a workplace with numerous issues, including some staff reporting "inappropriate behaviour" by MPs on a "regular basis".
Debbie Francis, QC, who was in charge of the investigation, said that unacceptable conduct was "too often tolerated or normalised".
In response to the probe's findings, Mallard ordered the creation of a new code of conduct for people who work within Parliament.
