The entire episode of Hercule Poirot: Third Girl will be available to watch here until August 1st, for those that missed it this past Sunday. Unfortunately, it is only available to those in the US.
Keep in mind, too, that Agatha Christie's Poirot: The Movie Collection, Set 5 - which includes the three episodes featured on Masterpiece Mystery this season (Murder on the ORient Express, Third Girl and Appointment With Death) - will be released next week on July 27th.