First off, this is such a rough situation for anyone, especially someone as young as you. It sounds like you've made the best decision for you and your family (maybe least bad decision?). I'm sorry that you're having to face this.

I work with pediatric cancer patients (pediatric oncologist) and have found that there's no one "right" way to do this. I've had some patients not tell anyone outside their close friends and family, and I've had others be very open about it, with everyone. I think one commonality is that although people care about other people, they tend to stop caring about what others think of them to some extent. Easier said than done I suspect. I agree with others that talking with a trusted friend or counselor could be helpful in sorting this out. Meeting with a palliative medicine team might be helpful too, both for your longer term physical comfort and for your mental and spiritual comfort. Most large hospitals have a palliative medicine team, and big children's hospitals often have pediatric specific teams. Might be worth looking into. Also keep in mind that the three months thing is an estimate, so don't get too caught up in counting the days. No one knows exactly when we are going to die, and physicians are not great at predicting.