The figure is lower by 0.5 percentage point than the government's estimate of 4 percent.
The growth rate for next year "could go down further, depending on the global economic situation," the Korea Development Institute (KDI) said.
The KDI also cut its growth forecast for this year by 0.3 percentage point to 3.4 percent, citing sluggish domestic consumption and investment.
The government expects the economy to grow 3.7 percent this year.
Last July, the International Monetary Fund (IMF) lowered its 2014 growth forecast for the global economy to 3.4 percent from an earlier estimate of 3.7 percent, citing one-off factors and slower demand in emerging markets.
The IMF forecast that the world’s economy will grow 4 percent in 2015.