More than 31,000 men in three countries have been bilked of millions of dollars by a scheme that promised they could retire to a paradise tended by nude angels, Federal prosecutors say.

Victims of the scheme were told they could retire to a ''valley paradise'' where they would be cared for and ''have all their wishes and dreams fulfilled'' by ''Mother Maria and the Angels of Love,'' prosecutors said.

Men who responded to an initial mailing were sent photographs of the nude or semi-nude ''angels,'' but the retreat did not exist and the pictures were of models hired by the defendants, investigators said. 'Too Good to Be True'

''These were lonely human beings who saw something too good to be true,'' said United States Attorney J. William Roberts, announcing an indictment Friday charging an Iowa couple and a 24-year-old Illinois woman with mail fraud in the case.