The attack on Saturday follows the murders of two soldiers and an official in the neighbouring Southwest region in two separate incidents.
Five other officials also kidnapped in one of those attacks were still missing on Sunday.
“Three gendarmes who were at their checkpoint were attacked and killed” by separatists, Ngoketunjia county prefect Quetong Handerson Kongeh told AFP, adding that two of the three killed had been decapitated. A fourth managed to escape.
He said a large number of rebels overwhelmed the gendarmes who “could not defend themselves”.
A bitter independence struggle by English-speaking rebels has been raging in Cameroon’s Northwest and Southwest regions since 2017.
Members of the anglophone minority in the country’s westernmost provinces have long complained of being marginalised by the French-speaking majority and 88-year-old President Paul Biya, in power for 38 years.
From 2017, their demonstrations devolved into a bloody conflict.
The rebels have extended their violent attacks against police and soldiers to civilians.
UN and international aid groups say both army troops and anglophone rebels have committed abuses and crimes against civilians.
In the past four years, more than 3,500 people have been killed and over 700,000 have fled their homes to escape the conflict.