All three were bought by an unnamed telephone buyer - believed to be a 44-year-old man from Kent who works in the music business - at the Sotheby's sale of sporting memorabilia held at Kent County Cricket Club's ground in Canterbury.
Mr Cooper, 59, who attended the bidding, sold the belts - each earned by successfully defending his British title three times - after losing heavily on Lloyd's insurance market.
He had hoped they would fetch pounds 70,000. But he said he was not disappointed at the amount raised.
'I am happy enough that they are staying together,' he said.
The first of the belts to go was the oldest and the only one made of gold. It was made in 1936 and won for the first time the following year by Tommy Farr. It was won outright by Mr Cooper in 1959 after he beat Brian London on points at Earl's Court Stadium. It was sold for pounds 22,000.
The other two, won in 1964 and 1967, went for pounds 10,000 each.
Mr Cooper, undefeated British champion for 11 years from 1959, is the only boxer at any weight to have won three Lonsdale Belts outright.