This might not be the right thread to bring this up, but I have been thinking about the animosity toward the unicorn hunter stereotype.
I am still relatively new to poly relationships and pretty bad at it, so I might not know what I'm talking about. You seem pretty down on the first scenario you propose, but with the exception of demanding the new person be equally into the first two, I don't understand why that's an unreasonable goal.
I guess I am sensitive to this because I think my partner and I dating someone together is probably the best arrangement for us, at least for now. I'd like to understand better why so many people in the poly community see this as tacky or short-sighted?
Or maybe you can elaborate on the distinctions between the unicorn hunters and serendipitous triad? I feel like the latter is more what we're shooting for, except if that's the goal at the outset, it doesn't meet the criterion of being serendipitous in the way you mean it.