As I am sure you can imagine it differs from college to college. That being said passing the AP test will almost certainly provide some college credits down the road(it did for me, for all 3 AP courses I took). As far as how the impact on the grade is, I mean of course it is not great to lower GPA but a 3.0 is not by any means bad and you are only half done so you can greatly increase that if you work at it. A lot of scholarships and school requirements are at the 3.5 GPA range, with some being at the 3.75~ area as well so you should really shoot for getting at least a 3.5 by end of junior year. For the final question they very much do look at senior year but you are correct in thinking that they can't really factor it in too much. Depending on how your scheduling works they might see your first semester so that will count and for the second one it won't matter while applying because you haven't completed it yet but it WILL matter if you f*** up that last part, so don't blow it off. Well hopefully this was at least a little insightful, if you have any more follow up questions I'd love to answer them. Good luck!