Yeah, I recently found out that a family I know (and have known for about 8 years) is in one. I literally thought that the third woman was the nanny! I'm a moron, apparently, because our other friends knew. I just thought they were very attached to the nanny.

I thought it was odd when there would be a dinner party and she'd show up with them but just minded my own business.
The husband is straight, the wife is bi, and the third woman is a lesbian.
I've not heard the term throuple, though. They refer to themselves as poly something. Not poly-amorous. It's poly and then something that means committed relationship, not casual sex.