I think your scores are fair. GPA is low, but can be saved by a higher GMAT score. I know that Texas MPA (comparable to USC) student's average GPA is 3.7 and GMAT is 670. If you retake the GMAT, aim for a 700 to really standout.
I like the economics degree, but the Biochem degree is unrelated and raises a red flag for me. For the essay, you should have a solid explanation of your degrees, why you didn't do accounting as an undergrad, and why you want to do accounting now. If I were reviewing your app, I'd be place extra scrutiny on your essay response relating to Biochem.
The extracurricular activities are good and could help you standout. I like the first 2 extracurricular activities, but the alt. spring break needs further explanation and mentioning Biochem again might hurt you. I've heard of alternative spring breaks (ASB) hosted by Deloitte that invited students to volunteer cleaning up after Hurricane Katrina. If this is similar to your ASB, then great because you can mention working alongside accountants. You need to make sure you explain the significance (did you have to apply to the ASB/was it prestigious) of your ASB, because spring break is only 1 week long. I think you could leverage the lab to show you have work ethic, but again, you do not want to place any extra emphasis on Biochem as it is unrelated to business/accounting.
Make contact with admissions/alumni/students to get an idea of what to put in your essay.
I've only researched the Texas MPA program which is typically ranked comparably to USC. I don't know anything about the other 2 schools except for their locations which makes me wonder, why did you pick these 3 schools? Their locations are allover the place. Are they top ranked and is ranking your only criteria?
You didn't provide much information about what you plan to do with a masters in accounting. I could help you out more if you provided more details. Without any other information, I'd recommend researching accounting career paths and talking with accountants about their career paths.
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GMAT Retake - Summer/Fall 2010 Minimum 650, Goal 700
Resources - Kaplan Premiere, MGMAT Guides 1-8, OG12, Quant & Verbal
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