Posting MTA cops at turnstiles in three major subway stations deterred virtually all fare-beating there, the agency said Monday.

Between Oct. 7 and Oct. 11, the MTA stationed cops at 34th Street-Penn Station, 42nd Street-Port Authority Bus Terminal and Brooklyn’s Atlantic Avenue-Barclays Center station, according to a study unveiled at the agency’s monthly board meeting.

While unmanned entrances in those stations saw anywhere from four to 46 turnstile-jumpers between the 7 a.m. to 8 a.m. rush, nearly every straphanger ponied up at the manned posts, the anecdotal reports found.

The Penn Station and Atlantic Avenue entrances saw no fare-beaters, while 42nd Street logged only two over four days.

Fare-evasion reports are on the rise, but the MTA in part attributed that increase to improved vigilance and data-collection.

The study was revealed amid debate over how best to clamp down on turnstile-jumping as the agency tries to justify the costly recent hire of 500 new transit cops to patrol subways and buses.

“Is it really necessary to have a fully qualified police person do this work?” asked board member David Jones, suggesting that the MTA have the equivalent of “meter maids.”

It was reiterated Monday that not all of the cops — hired at a minimum of $93,000 each — will be dedicated to scofflaw patrol, instead splitting evenly between trains and buses to probe more pressing issues, such as assaults on MTA workers and spikes in serious crimes.

But that division of labor also drew scrutiny, with board member Bob Linn noting that fare-evasion runs much more rampant on city buses than on the rails — so much so that the five worst bus routes hemorrhage as much cash as the 50 worst subway stations.

“The idea of putting half of the cops on the buses and half on the subways doesn’t make sense,” he said.

Larry Schwartz, a board member long allied with Gov. Andrew Cuomo, advocated for the return — and amplification — of the widely despised emergency-exit alarms to discourage riders from jumping turnstiles.

“I don’t know why it doesn’t sound like a fire alarm going off, where it’s near-deafening noise,” he said.