Disclosure: this comparison is published by SpicySwingers, which isn't one of the two platforms below. That's deliberate: for this specific use case, finding a single third person, we think the honest answer doesn't involve us, and it's worth saying so directly.
Why this comparison is different from a general platform comparison
Most lifestyle platform comparisons ask which is best overall. This one asks a narrower question: if a couple's specific goal is finding a single person, usually a woman, to join them for a threesome or an MFF dynamic, which platform actually produces results. The answer depends heavily on demographics that don't matter as much for couple-to-couple matching.
FabSwingers for finding a third
Fab has a large population of single people, including single women, within its overall membership. The lifestyle-specific framing works in your favour: everyone on the platform already understands the couple-seeking-third dynamic and its etiquette, which reduces the awkward education step. The forums and chat rooms also mean you can get a sense of someone's presence in the community before messaging, which some couples find reassuring.
The downside is competition and demographics. Single women on Fab, as on every lifestyle platform, receive high message volume and can afford to be highly selective. And Fab's overall membership skews toward an older demographic than Feeld's, which matters if you're hoping to connect with someone in their twenties or early thirties specifically.
Feeld for finding a third
This is genuinely Feeld's strongest use case. Its single-user population open to joining couples, particularly for MFF dynamics, is larger and more accessible than on any dedicated UK lifestyle platform, and skews younger. Feeld's culture is also more explicitly built around ENM and open exploration rather than the established swinging lifestyle specifically, which some thirds find a more comfortable entry point than a site with swinger in the name.
The downside is Feeld's lack of lifestyle-specific context. Because it's a broader ENM app rather than a swinging-specific community, some matches are curious rather than actively looking, and conversion from match to actual meeting is generally lower than on a platform where everyone has self-selected into the active scene.
Etiquette differences worth knowing
Fab's culture around finding a third has decades of established norms, and profiles seeking a unicorn are common enough that most single women there have seen every approach before, good and bad. Feeld's audience is less accustomed to being approached specifically as a third, and clumsy or objectifying framing lands worse there. The full guide to unicorn etiquette on this blog is worth reading regardless of which platform you use.
The honest recommendation
If you want someone genuinely embedded in the swinging lifestyle who understands the dynamic already: FabSwingers. If you want a younger, broader pool and are willing to do more of the explaining yourself: Feeld. Most couples serious about finding a third run profiles on both, since the two audiences barely overlap and neither is large enough alone to guarantee a match.
SpicySwingers is built for couple-to-couple connection rather than third-finding specifically, so for that particular goal we'd point you to the platforms above. For everything else in the UK lifestyle, create your free profile.