Nottinghamshire, Leicestershire and Northamptonshire will introduce the changes by early 2016.
The three forces already work together on major crime and forensics units.
Nottinghamshire Police and Crime Commissioner Paddy Tipping said some job losses were likely, but did not have an exact figure.
"We have reduced the number of chief constables, resource officers, we're going to continue to do that... but how many job losses? It's too early to say."
He said proposed budget cuts to policing would mean "a painful period going forward".
"It isn't a merger - but we are being asked to make savings of 25% or 40% over next few years," Mr Tipping said.
"We are determined to share senior officers - and I think this is a sensible thing to do."
A spokesman for Leicestershire Police said: "There is a strong desire to build on existing relationships by forming a strategic alliance, integrating services to protect the public."
The Police Federation said it was concerned the move might add extra pressure on remaining staff, adding its main priority was protecting frontline services.
Derbyshire has not joined the new system. It's Chief Constable Mick Creedon said: "One problem with proposed strategic alliances is that there is no immediate financial benefit.
"Forces need to adjust their budgets in the short term as well as for the future. Pinning hopes on alliances may not be the answer."
Three East Midlands' police forces have agreed to share some staff including assistant chief constables in a move to cut costs.