You probably know less about Social Security than you think.

More than two-thirds of older workers said they were confident about the rules of America's largest retirement program, according to a new survey by Fidelity Investments.

Yet among the 521 people age 55 to 61 that Fidelity polled in October, most got these three key concepts wrong:

Give advance notice

To that point, 65 percent of people close to retirement didn't know that you have to apply to the Social Security Administration three months before you receive your first benefits check. And 9 percent believed that the agency would contact them when it was time to receive benefits.