The Leicestershire, Nottinghamshire and Northamptonshire forces have asked G4S to carry out a feasibility study to see what it might be able to offer.
But Unison, the union for civilian staff, says it is "very concerned" about the move.
One current police employee said it was "disgusting to try to turn a profit out of policing".
Leicestershire Police's Assistant Chief Constable Phil Kay said: "The approach has been made to allow the three forces to gain information about what G4S might be able to offer in terms of service delivery, in areas such as contact management [999 and 101 call handling]."
He stressed that "no commitment" has been made at this stage.
Lincolnshire Police was the first force to outsource staff to G4S, in April 2012.
John Shaw, of G4S, said: "The improvements we've made at the control room in Lincolnshire have reduced response times and increased caller satisfaction, meaning Lincolnshire Police now has one of the top performing control rooms in the country.
"We're looking at how Nottinghamshire, Leicestershire and Northamptonshire may be able to achieve the same improvements."
However, Unison's Dave Ratchford said: "Putting policing in the hands of multinationals like G4S is a step towards a very worrying future."
"Justice is not and never, ever should be for sale," he added.
One police employee, who did not want to be named, told the BBC he was "very concerned".
"Privatisation is false economics and should never be applied to front-line public services. It is disgusting that we should have businesses trying to turn a profit out of policing," he said.
Three police forces are considering outsourcing their control rooms to private security firm G4S.