No score will guarantee you a spot at any school. You can see the median stats for those accepted to all the med schools on the School Selection spreadsheet stickied to the top of this forum. For California and other highly-selective schools great numbers aren't enough. You need to stand out in some other way too.
You've made a great start at some solid ECs. To improve your application, don't forget to shadow a few types of physician, get in a strong leadership experience, continue with the research, getting a publication if possible, and continue the terrific community service/clinical experience you're involved in. Another year will no doubt increase your cGPA and BCPM GPAs too. Even if they stay the same, I doubt that you'll be at much disadvantage with a very strong MCAT score.
1) I am planning to take my MCAT and apply next June so I wonder what score should I aim for if I want to get in USC, UC Davis or Tufts.
2) Anything else I can do to improve my application?
3) Would my GPA hinder my chances at California schools?
Thanks so much for your help.