"And according to a Navy casualty report released by the Kerry campaign, the third purple heart was received for "shrapnel wounds in left buttocks and contusions on his right forearm when a mine detonated close aboard PCF-94," Kerry's boat. As a matter of strict grammar, the report doesn't state that both injuries were received as a result of the mine explosion, only the arm injury."

Qoute from Factcheck.org

So for the third purple heart he got some rice in the ass and some shrap in the arm from a mine.