Objective: Threesomes, sexual acts involving three people, seem to become more popular. Their appearance in film, TV, press media, and blogs has increased, and there is even a mobile app facilitating such encounters. However, little to nothing is known how they come about, who is involved in them, and what sexual activities they include. This study´s goal is to explore these questions. Design and Method: An online survey will be distributed among users of 3rnder, the threesome app, participants with threesome experience from another study, the social networks of the researchers, and undergraduate students. It will assess experience with threesomes, preferences for them, and reasons for (not) engaging in them. For participants who have been involved in at least one, there will be more detailed questions about their most recent threesome: how it came about, who was involved in it, why they engaged in it, and what sexual behaviours it included. The response formats include tick boxes as well as open ended questions. Results: Results will provide the first picture of contemporary threesomes from a scientific perspective. Conclusions: This study will inform future investigations regarding the construal of sexuality and monogamy, and sexual behaviour more generally.