Firstly, don't get too "cart before horse" here. As
Polyglamorous points up, you don't mention whether they are a closed relationship. (No stats to back it up, but most that I've met are closed.) Then you have to figure out whether they are particularly interested in expanding their group, & to what degree. And THEN it's a question of whether they want to include YOU, as well as how centrally.
There's a little "poly math" that'll probably be mentioned, so I might as well take the lead.
When a couple "adds a third," it's not so simple as instantly expanding to make room for three, or even grafting a "secondary" onto a couple. Rather, A+B becomes A+B+C... & A+B, & A+C, & B+C (plus the three individuals, as individuality is often actively ignored in monogamy, but celebrated in polyamory). The tighter the group, the more effort must be put into maintaining ALL those dynamics.
It sounds a little like you have a bit more of a crush
on their situation rather than
on them. Nothing wrong with that, but the former motivating the latter will probably skew your reasoning. Don't chase a dream at the expense of a reality.
You probably cannot merely leap into the middle & instantly become a total equal. For starters, do the math: you (D) would have to simultaneously give YOUR FULL ATTENTION to growing these dynamics: A+D, B+D, C+D, A+B+D, A+C+D, B+C+D, & A+B+C+D
...as well as having enough "alone time" to keep your individualism properly maintained so that you'll be a full contributing member of the group.