There's no limit to how many resits you can do. I would retake your C's and D, however only you know how much you can handle. Also,take into account how many A2 exams you'll have. I got BBCC at AS, just about able to manage retaking 5AS exams along with 7A2s and cw. I got AAA (close to A*s in two of my subjects) and all a's in my modules apart from 2 A2 exams because I lost morale and time to revise for them at the end. If I had taken another exam I think it would've been much worse. Talk to your parents/friends/teachers and mull it over. I think 3 is pretty manageable though, especially if you're using AS knowledge in your A2 exams as you'll have to revise them anyway