The Spanish nationals were arrested at their home in Sheffield, northern England, in April 2007, days after a European arrest warrant had been issued for them by Baltasar Garzon, the investigating judge in Madrid.
In a statement issued on Tuesday, London's Metropolitan Police said: "Today three people wanted by the Spanish authorities in connection with alleged membership of terrorist organisation Eta have been extradited to Spain.
"Their extradition was ordered on August 17 and following all appeals having been exhausted the three were extradited on a flight that left at 1200 GMT today."
In a separate development, Spanish police have announced that they found 125kg of bomb-making material in a cache discovered following the arrest of two suspected Eta members.
The discovery, found in the northern province of Huesca on Tuesday, also contained detonators and timers, an interior ministry spokesman said on condition of anonymity.
The hidden materials were traced after police found a sketch map on one of the two people who were detained on Sunday.
The two suspects, identified as Igor Portu and Martin Sarasola, had been arrested at a security checkpoint in the northern Basque town of Arrasate.
Portu was hospitalised in San Sebastian early on Monday.
Alfredo Perez Rubalcaba, Spain's interior minister, said Portu was injured while resisting arrest.