Most Helpful Guy
I know lots of cases where that has worked out really well, personally. But usually with older people, those in their late 20s at the young end, up into 30s and 40s. I'm sure there are cases older as well, I just don't know any personally. It takes a LOT of emotional maturity to pull it off, to be able to work past jealousy and push it into compersion instead.
I'm not saying it can't be done, I'm sure that it could be, with the right people, but it will be harder for you at a young age. The biggest problem is something you don't really have any control over. The prefrontal cortex does not finish developing until the late 20s. Before that happens, a person's personality is really in flux. Things can change a fair bit, and long term planning is a lot more difficult (that is the region of the brain that really controls long term thinking). So at 18 and under, you will be trying to pull something off that 'adults' can struggle with, while dealing with parts that are not fully finished. Kind of like driving a car with three tires.
If you do decide to do it, I suggest all three of you promise to remain totally open and honest with each other. As soon as you start keeping secrets from each other, things will disintegrate incredibly quickly. You'll want to have all three of you there to talk about what you each expect out of this, what kinds of boundaries are acceptable, all that stuff. This will have the possibility of really going to shit fast, so you'll have to be vigilant in keeping communication open and flowing if you are going to make it work.
Most Helpful Girl
I find open relationships to be stupid because ultimately when the time comes you're going to choose one person and the other will be left in the dust with a broken heart.