The American Community Survey is the only source for a wide range of important statistics (including poverty and income date) down to the community level for all areas of the United States. It is an ongoing survey administered by the United States Census Bureau, and supplements and updates the demographic data gathered by the Decennial Census. The ACS that spans five years of data collection, covers all geographic areas down to the block-group level.
As of 2016, the ACS 3-year estimates have been discontinued by the Census Bureau due to budgetary constraints.The 2005-2007, 2006-2008, 2007-2009, 2008-2010, 2009-2011, 2010-2012 and 2011-2013 ACS 3-year estimates will remain available to data users, but no new 3-year estimates will be produced. This is a real loss of information for smaller geographic areas or specific sub populations.
If you want to compare one variable across a variety of places, you need to find the survey that includes the smallest size geography you want to compare. For example, if you want to compare the poverty rate in Bayview, CA (population under 2,400) and Richmond, CA (population 104,000), you need to use the ACS that is available for both geographies -- the 5 year estimate. 
DON'T compare the 3-year to the 5-year; and don't compare overlapping periods, for example, the 2005-2007 ACS 3-year estimates to the 2006-2008 ACS 3-year estimates.
