Sounds like an interesting situation!
I'm new here too, and welcome~!
I have some experience being the third for a couple. It's different in that we're in our 20s, it was more about friendship and sex and we live close. But there were a few things that made me uncomfortable and insecure, so maybe that'll help you.
I knew that if I caused conflict in their relationship, they would stop seeing me.
They were uncomfortable with me being emotionally attached to either them.
Their room, their house and their world were their own and I was very much a visitor.
I sometimes felt that I couldn't communicate what I wanted or needed to. Where as they were practically telepathic with each other.
The girl would decide that they were going to stop the sexual part of our relationship, and then not tell me or explain.
To sum up, they prioritized their relationship over they one they had with me. Which was understood, and the arrangement, but ultimately not one that continued.
Your situation sounds a lot more wonderful! Haha. This was more like a friends with benefits situation. But these are the fears that I had, being with a couple. Maybe you'll feel secondary in their house, or their lives. They're well established. Moving to a place where they know everyone and you don't is quite the leap.
But it sounds exciting! I would say be a bit wary just because living with people full time is very different to spending 2 weeks with them. Your living situation would be dependent on your relationship with them. I'd have a plan B. But maybe that's not an issue for you, financially or whatever. Just a thought.
As far as the rest of the world, going against the norm is always challenging, but if no one ever did, things would stay the same. if you're secure in yourself and your situation, it can just run off you like water. It's like when some stranger calls you a lesbian c*** and, if you're secure in your sexuality, it doesn't seem to matter (yes, that's from my own experience haha). Friends can be educated; people who mind don't matter and people who matter don't mind =).
As far as getting hit on, I spent a long time giving excuses that were 'kind of' true- "I'm in a relationship" (I'm in an open relationship) "I'm gay" (I sway more towards girls). Eventually I've just gone with "I'm sorry, I'm not interested." or "I'm not attracted to you." Which you can soften with a choice "I think you're a great person" or "(Aesthetically,) I think you're good looking but". I've never had a bad response. All you owe to people is to be clear =)
pansexual femmeromantic genderqueer person. pronouns: them/they