The three women claimed they were given enhanced privileges in return for sex. Two of them actually believed they were in love with the officer.
Acting on a tip-off, prison bosses installed secret video equipment to gather evidence over many weeks.
Detectives were then called into the all-women Bronzefield prison, in Ashford, Surrey.
The officer was arrested at his home on suspicion of misconduct in a public office. He has since got bail.
He was sacked and Surrey Police is conducting a criminal investigation.