A new law banning so-called upskirting has come into force in England and Wales.
Under the Voyeurism Act, offenders face up to two years in jail and being placed on the sex offenders register.
All three incidents reported to Hertfordshire police in 2018 involved allegations that a man was seen taking pictures up a woman's skirt.
In two cases, police were unable to identify the victims, while no suspect was identified in the other.
In all cases, it meant police were unable to prosecute, according to Freedom of Information Data from the Press Association.
The law comes into force following a high-profile campaign led by writer Gina Martin, who spent 18 months fighting for it to be made a specific offence after two men took a picture up her skirt at a festival in 2017.
Campaigners have claimed the lack of a specific upskirting offence has deterred victims from coming forward.
Ms Martin said: "During the 18 months of campaigning undertaken, I received hundreds of messages and stories from those who had been upskirted.
"It was obvious that we didn't have the tools to adequately paint a picture of what a big problem upskirting is, so I'm delighted that the Press Association has taken on the challenge of obtaining the first official stats on reports.
"We hope that people continue to feel comfortable reporting upskirting under the new Voyeurism Act."
The Ministry of Justice said the new law "bans the degrading practice to deter perpetrators, better protect victims, and bring more offenders to justice".
Justice Minister Lucy Frazer said: "We have always been clear - there are no excuses for this behaviour and offenders should feel the full force of the law. From today, they will."