How many U.S. First Ladies have been married more than once?

Frances Cleveland married Thomas Preston jr after Grover's death.
http://www.whitehouse.gov/about/first-ladies/francescleveland

Edith Wilson was married to Norman Galt, a jewelry store owner. She married Woodrow 7 years after Galt's death.
http://www.firstladies.org/biographies/firstladies.aspx?biography=29

Jackie Kennedy married Aristotle Onassis after the presidents assassination. She outlived him.
http://www.firstladies.org/biographies/firstladies.aspx?biography=36

From what I could find, those three were the only First Ladies to have been married more than once.

The first First Lady, Martha Washington.

https://en.wikipedia.org/wiki/Martha_Washington

Betty Ford was married and divorced before marrying Gerald Ford.

https://en.wikipedia.org/wiki/Betty_Ford#Early_life_and_career

I'd like to add:

"Florence "Flossie" King DeWolfe, a divorcee with one son, married Warren Harding, five years her junior in 1891."

"Rachel Jackson, Andrew Jackson's wife was a bigamist, having married Jackson before she was divorced from her first husband. She died after Jackson was elected president but before his inauguration."

http://www.firstladies.org/didyouknow.aspx

Dolley Madison was John Todd's widow.

http://www.firstladies.org/biographies/firstladies.aspx?biography=4