Third Girl is a work of detective fiction by Agatha Christie and first published in the UK by the Collins Crime Club in November 1966[1] and in the US by Dodd, Mead and Company the following year.

Mrs Oliver trails David, ending up in the hospital after being coshed on the head upon leaving his art studio.

Andrew returned to England after his brother Simon died a year earlier, to work in the family firm, arriving with a new young wife, Mary.

The main tenant, Claudia, is secretary to her newfound father; the other girl, Frances, travels often for the art gallery that employs her.

Sir Roderick engages Poirot to find documents missing from his files which brings young Sonia under suspicion.

He had David paint portraits of him and his late wife in the style of a painter popular 20 years earlier as part of the ruse.

The woman posing as her stepmother was also Frances, who used a blonde wig to cover her dark hair when changing roles.

Maurice Richardson in The Observer of 13 November 1966 concluded, "There is the usual double-take surprise solution centring round a perhaps rather artificial identity problem; but the suspense holds up all the way.

"[5] The novel reintroduces Stillingfleet, a character from the short story The Dream and first published in book form in the UK in The Adventure of the Christmas Pudding in 1960, and Mr Goby, whose previous appearance had been in After the Funeral in 1953.

In Chapter 4, while Poirot is pretending he shares a military history with Sir Roderick, he makes reference to Colonel Race from novels such as Death on the Nile and Cards on the Table, as well as Inspector Giraud from Murder on the Links.

As is true with most of the later novels adapted for Agatha Christie's Poirot, the time period is kept vague rather than making it distinctly 1960s.

The novel was also adapted as a 2017 episode of the French television series Les Petits Meurtres d'Agatha Christie.

In the US a condensed version of the novel appeared in the April 1967 (Volume 128, Number 6) issue of Redbook magazine with a photographic montage by Mike Cuesta.