The U.S. dollar index fell to a three-year low Wednesday and looks set for more declines after Treasury Secretary Steven Mnuchin said a weak dollar was good for U.S. trade.

The dollar index is now down 10 percent in the past year and broke below the key 90 level after Mnuchin's remarks. According to Kensho, there are some big winners in the Dow when the dollar is falling.

CNBC's analysis looked at when the dollar index is down 5 percent or more in a three-month period. That happened 29 times since January 1993.