The unheralded golfer from Adelaide fired an impressive five-under-par 67 to lead on 11 under.
Cullen opened a four stroke lead over compatriot Stephen Leaney, in the season-opening event, which is jointly sanctioned by OneAsia and the Japan Golf Tour Organisation.
Multiple-tour winner Leaney, bidding for a comeback in the game after illness, also shot a 67, while American David Oh carded a 71 and is a stroke further behind.
David Smail from New Zealand, the leader after the first and second rounds, slipped back with 74 and is five under. Two double bogeys on his card proved costly.
Cullen, the 27-year-old who turned professional in 2009, made six birdies and dropped just one shot, playing with a confidence and demeanor that suggests he is a seasoned competitor at this level.
But the Australian said there was no secret to his fine form.
“I am not even thinking about it I am just playing golf,” he said.
“I have been practicing hard and I have put in a lot of hours the last few weeks. As long as I put in the work before I come away for a tournament, I know I am ready.”
Earlier this year Cullen qualified for the Open Championship after making it through international qualifying on home soil and that form seems to be continuing as the year progresses.
“I hit it pretty solid but not as solid as yesterday, but I only missed a couple of fairways," said Cullen.
Cullen said he was giving himself chances to hole putts, but could do better.
"I think I am giving myself too many downhill putts. It is hard to hole downhill putts as you cannot be aggressive."
The relative newcomer to the game is being chased by one of his country’s most successful golfers.
Leaney has won seven times on the PGA Tour of Australasia and has been victorious on four occasions on the European Tour. In 2003 he hit the headlines after finishing second in the U.S. Open.
Unfortunately in 2008 he was diagnosed with vertigo and he has struggled to recapture his best form since then.
The 43-year-old had three bogeys, but eight birdies -- including two over the last four holes -- giving him a great chance for an emotional comeback victory.
Oh was delighted to find himself in the top three having aimed just to make the cut ahead of the tournament.
“I’m a little bit nervous as I haven’t been in this position for a long time,” said the California native, making his first start of a season in which he aims to play as much golf as possible on OneAsia and the Japan tour.
Asked what his strategy would be for the final round, Oh said: “I just want to be as steady as I can. Obviously Nick has a good lead and I wouldn’t wish a bad round on anyone, but anything can happen on this course.”
Smail, a five-time winner on the Japan tour, started the day with a one stroke lead from Cullen.
However, he got off to the worst possible start with a double bogey seven on the par five first. He steadied the ship with THIdies later on, but then suffered another double on the par-three 16th.
Indonesia’s top golfer Rory Hie continues to lead the local challenge after carding a 72 to lie one under for the tournament.
Hie, who finished second in this event last year, was in position to finish the day closer to the leader but disappointingly double bogeyed the final hole.
Defending champion Thaworn Wiratchant from Thailand is on the same score after returning a 69.
Par 216 (3x72)
205 - Nick Cullen (AUS) 72 66 67
209 - Stephen Leaney (AUS) 72 70 67
210 - David Oh (US) 72 67 71
211 - Ashley Hall (AUS) 70 74 67, David Smail (NZ) 68 69 74
212 - Park Eun-shin (KOR) 74 69 69, Mamo Osanai (JPN) 71 72 69, Hu Mu (CHN) 73 68 71
213 - Anthony Summers (AUS) 75 71 67, Hiroyuki Fujita (JPN) 70 71 72, Yoshikazu Haku (JPN) 70 69 74
214 - Ryu Hyun-woo (KOR) 75 71 68, Terry Pilkadaris (AUS) 73 71 70, Jay Choi (US) 71 72 71, Kazuhiro Yamashita (JPN) 70 73 71, Daniel Fox (AUS) 73 70 71, Liang Wenchong (CHN) 69 73 72, Kim Meen-whee (KOR) 70 70 74
215 – Thaworn Wiratchant (THA) 75 71 69, Scott Strange (AUS) 73 73 69, Prom Meesawat (THA) 74 71 70, Rory Hie (INA) 73 70 72