Documents seen by the Irish Examiner show the hike was signed off by the council’s corporate policy group, which claims it will mean an average increase of just €1.50 per week.
The rent increase will bring in an estimated €750,000 per year.
Tenants will be written to in the coming days informing them of the move.
The council is to carry out a countywide review of tenants’ incomes.
This will get under way in March and will determine how much they will pay based on the household income.
Sinn Féin councillor Des O’Grady said the household income review should have been carried out before any across the board 3% rise.
“We are totally opposed to these unnecessary rent increases during the present housing crisis in the context of continually decreasing standards of maintenance and the poor service many tenants are seeing for their rents, as well as the fact that Cork County Council is failing to maintain its properties on an ongoing basis,” said Mr O’Grady.
He said the latest rent hike will impact on all council tenants and cause severe hardship for many families across the county.
“Low-income families who suffered a drop in living standards in recent years simply do not have extra disposable income and they cannot afford a rent increase of any amount,” he said.
“A 3% increase may seem very little to those earning huge salaries but this rent increase could drive people, who have already suffered years of austerity and are only scraping by financially, into homelessness.”
He said the whole process is doubly flawed as the council is first introducing a blanket 3% increase “in a ham-fisted manner and afterwards carrying out a review of current rents”.
Mr O’Grady maintained this is totally unjust as many families may end up paying unwarranted higher rents while this review is taking place.
“Will they be able to claim back the money, I doubt it,” he added, describing this as “inexcusable” as it is coming at a time when the council has amassed an extra €3.3m income in 2016.
This is made up of a €2m increase in local property tax (LPT) and an extra €1.3m in commercial rates from new businesses.