I had to look this up, so I figured I might as well share.
Mark Loretta had previously homered in consecutive games three times in his career, the most recent occasion being in 2004.
While playing for the Padres, Loretta homered at home vs. the Braves on the 18th of August, then did it again in the Pads' next game on the 20th, again at home vs. the Marlins.
He has never, of course, homered in three straight.