You're right. But for 3,000,000 votes cast, the margin of error is about 6 hundredths of one percent. At the 99% confidence level, it's .074% variance.
http://en.wikipedia.org/wiki/Margin_of_error#Different_confi...
In reality, you should apply a finite population correction, which would make the "margin of error" a bit smaller if you get a substantial percentage of the population voting:
http://en.wikipedia.org/wiki/Finite_population_correction#Co...