The caches are located in the departments, or counties, of the Gers, Pyrenees-Atlantiques and Hautes-Pyrenees, they said.
ETA announced earlier this week that it would hand over all its remaining weapons by Saturday, a move bringing the final curtain down on its armed campaign for a Basque homeland.
The group, founded in 1959, has been blamed for the deaths of 829 people in a string of bombings and shootings dating back to 1968.
In 2011, it announced that it had abandoned its armed campaign.
It has more recently sought to negotiate its dissolution in exchange for amnesties or improved prison conditions for roughly 350 of its members held in Spain and France, and for current members living under cover.
But both France and Spain have taken a firm line and refused any concessions.
Experts on ETA have previously estimated the group's remaining arsenal to comprise 130 handguns and two tonnes of explosives.
On Friday, a Basque environmentalist named Txetx Etcheverry, apparently acting as a go-between, told AFP that the weapons would be checked on Saturday by an outside verification body.
The panel, which includes a former Interpol secretary general, Raymond Kendall, is not recognised by either the French or the Spanish governments.
French police are on standby to take possession of the weapons, officials have told AFP.
An event is being planned in the French Basque city of Bayonne on Saturday afternoon to mark so-called "Disarmament Day."
In Madrid, the government on Friday dismissed ETA's disarmament as a unilateral affair and warned that the group -- which it denounces as a terror organisation --- could expect "nothing" in return.
"It will not reap any political advantage or profit," Inigo Mendez de Vigo, Spain's culture minister and its government spokesman, said.
"May it disarm, may it dissolve, may it ask forgiveness and help to clear up the crimes which have not been resolved."
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