The 1099 SSA statement that you receive in January as your tax document will show how much as received in payments for 2008, 2009 and 2010. You can then take each of these amounts to determine how much would have been taxable if you received it in the appropriate year. You then report the total for the 3 years as the taxable amount on the 2010 tax year.
For example if you receive SS of $4000 for 2008, $11,000 for 2009 and $11,000 for 2010:
Calculate how much of the $4000 would have been taxable on the 2008 return (lets say $1000). How much of the $11,000 would have been taxable on your 2009 tax return (lets say $6000). How much of the $11,000 would be taxable on your 2010 tax return (lets say $5000). You would then report $12000 of taxable SS on your 2010 tax return and pay the tax.
FYI - for a single individual SS will be taxable if other income plus 1/2 of the SS is over $25,000 ($32,000 for MFJ). Up to a maximum of 85% of SS can be taxable depending on how far over the limit you go.
Hope this helps. If you still have a specific question, please hit the need info button and ask. If you are satisfied with the answer please hit the green accept button to close out the question and allow me to receive payment.