A total 97% of three-year-olds already enrolled in nursery classes.
President Emmanuel Macron was expected to formally announce the decision, which will require an additional 800 school staff, during the inauguration of the Assises de la maternelle in Paris on Tuesday.
It is the latest in a string of reforms to the education system in France during Mr Macron's presidency, which have already seen CP class sizes cut in targeted disadvantaged areas, modifications to university access procedures, reforms of the bac and vocational training.
The current age for compulsory education is six, when children enter cours préparatoire (CP) classes. The change to the rules is expected to affect some 26,000 children, as 97% of all youngsters aged three and over are already enrolled in schools.
In a statement, the Elysée said: "The decision to lower the age of compulsory education to three years reflects the will of the President of the Republic to make school the place of real equality and a recognition of nursery school, which should no longer be considered as a universal mode of childcare or as simple preparation for elementary school."
Comment: By 'a place of real equality' do they mean a place where all of these toddlers can be fully indoctrinated with the values of the state as opposed to the values of their parents? Can't have those pesky parents being responsible for their own child's development, can we?
Mr Macron is also expected to announce a reform of initial and ongoing training of kindergarten teachers and teaching assistants.
Comment: Numerous studies have stated that children should not be separated from their parents for any length of time before age 9. Age 9 is also the ideal time for a child to begin formal schooling.