This money will grow tax-deferred: that is, you'll receive a tax deduction today and won't pay taxes again until you withdraw the money in retirement.

If we take the two intermediate-return scenarios, not only would you end up with $3 million, you would end up with far more. This is simply due to the power of compound interest applied over long periods of time. In other words, the sooner you start investing money in tax-advantaged vehicles, the sooner you'll see a runaway compounding effect.