Why not go for friendship first? If you are looking for a relationship and would love something longterm to develop, get to know her. You don't have to jump into it and have all the time in the world. Get to know her, establish a good friendship and maybe things will be sorted out by themselves. It would take some pressure off of the situation, because getting to know someone new while already planning your shared future can be quite stressful. Don't put yourself or her under too much pressure.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.