Hi, greenbasil, I'm new here, too!
My only suggestion is to make sure you spend some time with just her, even if it's just a meal or so. If you really want a triad relationship, I can't imagine it would work if the only thing you have in common is him.
I also really like the suggestion of living near but not with, at first. Be a part of their lives, see how they work on a day-to-day basis. I think that's just smart when you're looking to cohabit with someone, romance or not.
I hope you get centered and then have a fabulous time in two weeks. Best of luck!