Three-year average monthly returns of the Standard and Poor's 500 Index can be found at YCharts. The index's three-year return for May 2015 was 60.83 percent, compared to 49.19 percent for April 2015 and 43 percent for May 2014, according to the site.Continue Reading
More recent data can be found on Morningstar's website. As of July 24, 2015, the average three-year return of the S&P 500 Index stood at 18.31 percent. The site also displays returns over a period of a week, year-to-date, four weeks, 13 weeks, one year and five years.
The S&P 500 Index is composed of stocks of 500 U.S. companies representing the broad U.S. economy. Together, these companies' stocks reflect about 80 percent of the U.S. market's capitalization and the index is therefore considered a bellwether of the U.S. economy, explains The Motley Fool. As of July 2015, the biggest company in the index is Apple, which is worth at least $700 billion.
The three-year return of a stock or index is the total compounded return over the past 36 months, defines Absolute Returns. As stock indexes can bring unusually high or unusually low returns on investment over a short period of time, the three-year return presents a snapshot of how an investment in the index performed over that time period, explains About.com.Learn more about Financial Calculations