I don't think its gonna work out. You've agreed to wait for 3 months before having sex instead of just going with the natural flow of a relationship. So one of many options is gonna end up happening.
1) If you run into relationship problems and arguments you both will just trudge through it for 3 months just in order to get to the final shebang. And if you don't you'll end up being resentful of it.
I think this will most likely be the outcome. I don't know how long relationships now a days last at 15, but 6 years ago (when I was 15) they didn't last very long. A couple months max. A majority never making past 12 months.
But with ya'll agreeing on a set time to have sex, it's going to seem akward (sp). You may/will run into arguments are road blocks in your relationship but going to look past it just for sex. And that may not even be that good. Then you may end up regretting what you did. I regret losing my virginity to the girl I lost it to. But I was feeling down that day and my aunt was there to comfort me. One thing led to another.....
Anyways, you should have just let it come natural instead of setting a time and date when you can have sex. I can understand if you say your not ready until after 3 months, but by the way it sounds, is like right when the 3 months is up, it's fucking time.