I guess by definitions here, I am a unicorn. The way we look at it is we have a relationship that contains three of us, two of us are married. Could talk endlessly around how we make things work, how we deal with things when they don't work and how we try to match expectations. but guess that isn't the question here.
We've been together for two years on 2nd August, but known each other for about 12 years and been very good friends for all that time. We joke that we've 'dated' for 10 years before we got into bed. I can't say our sex/romantic relationship developed naturally out of our friendship. There was a very conscious line in the sand that we crossed at that point. However, I/we do feel that our relationships have evolved to the point they are at now and we do work on them. However, i do think the fact we were friends first and have similar beliefs/ideologies is the main reason our relationship works. They did not go out looking for me to 'fit into their relationship'.
For sleeping arrangements, pretty much as soon as we got together we've all wanted to sleep together. However, we all also like to have our own space to sleep. so, we have pushed a single bed and a double bed together to create an 8ft wide bed. We have one of these in both their house and at my house. When we stay at hotels, where possible we push the double and single together. If not possible, usually my male partner has the single bed, because he's driving, but if one person is ready to crash before the others, they take the single.
We have a double duvet and single duvet across our huge beds. this means we all get an edge of the duvet to fling off and round as required.
I currently live with my partners in their house (referred to as theirs to distinguish for the purposes of this post - we normally refer to both houses as 'ours') for work (my house is 200 miles away). I work 4 days a week and leave the house very early in the morning, before either of them even have to be awake. For that reason, I sleep on one side of the bed. My male partner sleeps on the other side of the bed (for personal reasons) and my female partner in the middle. In either house, on nights where I do not have to get up in the morning, I sleep in the middle. There are rare occasions when my male partner will sleep in the middle, but they are few and far between. And there are some nights where we all just sleep where we fall
I personally like sleeping with both of them and wouldn't now want to be in a separate room from them, however, we are not all squashed into one double bed, which would be a deal breaker for me. We each have just short of 3ft of space to sleep in.