Using Multiyear Estimates addresses Interview Collection Periods, Geographic Boundaries, Weighting Methodology - Use of Population Estimates; and Inflation Adjustments. Re: the latter, "Monetary values for the ACS 1-year, 3-year, and 5-year estimates are inflation adjusted to the final year of the period. For example, the 2005 - 2007 ACS 3-year estimates are tabulated using 2007 adjusted dollars. These adjustments use the national Consumer Price Index (CPI) since a regional-based CPI is not available for the entire country."
For a more comprehensive list of ACS subject definitions, go here.