I haven't grown these yet myself, but hear they are fairly easy to grow, supposed to be similar to growing tomatoes (same family), so it could be your seed if you haven't done anything wrong (under watering, over watering, heat issues etc). I should think a starter mix would be best. Did you literally plant 3 seeds, and 2 come up, as that doesn't sound that bad. I don't know what your weather is like in Colorado, but I know these do want some heat, so that could be the problem and would also indicate whether you could sow directly in your raised bed. In the Uk people tend to grow them in a green house. If you have a decent amount of seed I would plant more than a few for better odds, and could try some outside just for the hell of it. I hope thats of some help. Good luck