The 40-year-old carded a 72 for a 10-under tally of 206, one behind Englishman Anthony Wall (67) and two adrift of leader Jamie Donaldson of Wales (69).
Harrington is without a tour win since the 2008 US PGA Championship. Donaldson believes the Irishman will be a major threat in the final round, if not quite his biggest danger.
"Padraig is a class player but it's me versus me really," the Welshman said as he bids to land his first trophy in 254 career starts.
"The only person I am playing against, I suppose, is myself. I've got to stay out of my own way."
Harrington, who shares third spot with England's Mark Foster (73), has waited a long time to get back in the winners' circle and his level-par round in the wind and rain at Royal Portrush in County Antrim left the rest of the field wondering if Sunday might be his day.
"You might think I am the form card but we are all going to have our little demons out there," said the 2007 and 2008 Open champion.
Harrington, who also won the Irish Open in 2007, believes there are pros and cons in having the backing of a home crowd.
"You've got to use the home crowd in your favour," he said.
"You've got to let them carry you when you need to be carried and don't get carried away by them.
"If you feel like you're letting them down, you have to fight that urge. This ain't no exhibition, I can guarantee you that.
"There's nobody going out there tomorrow thinking, 'We'll let the Irish guy win'. It doesn't work like that," added Harrington.
Donaldson has had 32 top-10 finishes without achieving a breakthrough victory.
"I've had a few chances but at the end of the day I've not been good enough so far to stand on the last green holding the trophy," said the 36-year-old.
"You've got to keep trying and it's one shot at a time, one hole at a time."
World number two Rory McIlroy will need to produce a spectacular final-round charge to revive his title chances after a 71 left him six strokes off the pace.
McIlroy's fellow Northern Irishman Graeme McDowell carded a 73 for 212 while British Open champion Darren Clarke returned a 73 for 213.