The Dublin native shot 1-under-par 71 in the third round for a total of 4-under 212.
Through three rounds, the Dublin native has been better able to handle the strong winds that have plagued most of his challengers. He is trying to become the first Irish winner since John O'Leary in 1982.
"My goal was to go out there and basically win the tournament today by maybe shooting 66 and running away from the field," Harrington said. "I was a little disappointed, but whether I'm three shots ahead or four shots ahead it won't make much difference tomorrow."
Bradley Dredge of Wales (69) is tied for second with England's Simon Wakefield (73) at 1-under 215.