Does somebody know about three years law (of taxes)?

Federal law requires you to maintain copies of your tax returns and supporting documents for three years. This is called the "three-year law" and leads many people to believe they're safe provided they retain their documents for this period of time.
However, if the IRS believes you have significantly underreported your income (by 25 percent or more), or believes there may be indication of fraud, it may go back six years in an audit.

To be safe you must keep the following personal documents for 6 years: Supporting Documents For Tax Returns, Accident Reports and Claims, Medical Bills (if tax-related), Sales Receipts, Wage Garnishments, Other Tax-Related Bills.
You also must keep the following personal documents for ever: CPA Audit Reports, Legal Records, Important Correspondence, Income Tax Returns, Income Tax Payment Checks, Property Records / Improvement Receipts (or six years after property sold), Investment Trade Confirmations, Retirement and Pension Records.