The 47-year-old Prayad, who led by two overnight, went out with a flawless 31 at the Alpine Golf Resort-Chiangmai before shooting three more birdies against two bogeys to put some daylight between him and the chasing pack with a 18-under-par 198 total.
His compatriot Thongchai Jaidee, a three-time Asian Tour number one, battled to a 69 for tied second place but conceded he needed a miracle to topple the runaway leader for a win which he needs to break into the world's top-50 and earn a place at the Masters Tournament in two weeks' time.
Australian newcomer Matthew Stieger shot a fine 68 as he remained bogey-free for the week while Asia's first Major winner Y.E. Yang of Korea carded a 67 for a share of fourth place on 204 with South African Anton Haig (66), Kiradech Aphibarnrat of Thailand (66), Indian Digvijay Singh (70) and Australian duo Marcus Both (65) and Scott Hend (67).
Four-time Major champion Ernie Els of South Africa fired his lowest round of the season with a 66 to move up to tied 15th place, eight shots behind the leader.
In-form Prayad, who has won three times at home this year including his national Open a fortnight ago, is chasing his seventh Asian Tour title but his first in nearly six years on the region's premier Tour, which is celebrating its milestone 10th season in 2013. A chip-in eagle on seven proved to be the highlight of his day as he held on to the lead for the third straight day.
"I have a five-shot lead now and I will play steadily and not be aggressive. I think the pressure is on those chasing me. They need to have a good front nine or else they won't be able to catch up with me," said Prayad.
The smooth-swinger felt the effects of the heat and humidity over Alpine and stumbled briefly with bogeys on 15 and 16 before bouncing back with a birdie on 17. "I made mistakes but it doesn't matter. I was careless on 16 but I still think I have a very strong advantage," said Prayad. "I felt tired because it was quite hot. But I feel very comfortable on this golf course because I like the landscape a lot. I like playing on golf courses with nice scenery."
Thongchai, who needs a win on Sunday to have a chance of qualifying for the Masters, sank five birdies against two bogeys to keep alive his slim hopes of lifting a 14th Asian Tour victory. "I think the problem was my putting and reading the lines," said the three-time Asian number one, who needed 30 putts.
"I hit a lot of good shots but the greens are killing (me). They are tough to read. Prayad played so well. He dropped a couple of shots but he came back. He likes the course and he's in form. He's got a good chance. I'll try my best but I'll need luck tomorrow [Sunday]," added Thongchai, who is ranked 59th in the world.
Stieger, playing in his rookie season on the Asian Tour, was delighted to keep the bogeys off his card at the Chiangmai Golf Classic, the first Asian Tour event in the northern city of Chiangmai.
"I'm pretty amazed I'm through 54 holes without any bogeys ....touch wood! Hopefully we can keep that going. The last three days, I've stayed really patient and had some good chances," said 22-year-old.
Like Thongchai, the powerfully-built Yang, who is an Asian Tour honorary member, was also mystified by the tricky Alpine greens, needing 28 putts for his round. "I could have scored lower. I missed a lot of short putts which will probably haunt me," said Yang, who won the U.S. PGA Championship in 2009.
"It's confusing some times. Some greens look like it's going right but it doesn't necessary mean so. I'll try to get to four or five under on the front nine tomorrow [Sunday], try to put some pressure on Prayad. Sometimes, a six-shot difference can change very easily."
Both, a two-time Asian Tour champion, produced the joint low round of the day as he stormed home with an eagle and three birdies over his closing five holes. "I'm not going to be disappointed with a 65 but I was playing with Pariya (Junhasavasdikul) and he reminded me that I should have been a few better! I'm sure I've got a bit of work tomorrow [on Sunday to catch Prayad]," said the 33-year-old Aussie.
Scores after round 3 of the Chiangmai Golf Classic presented by PTT being played at the par 72, 7,471 Yards Alpine GR course (a- denotes amateur):
198 - Prayad MARKSAENG (THA) 65-67-66.
203 - Matthew STIEGER (AUS) 68-67-68, Thongchai JAIDEE (THA) 69-65-69.
204 - Marcus BOTH (AUS) 71-68-65, Anton HAIG (RSA) 72-66-66, Kiradech APHIBARNRAT (THA) 71-67-66, Scott HEND (AUS) 68-69-67, Y. E. YANG (KOR) 69-68-67, Digvijay SINGH (IND) 67-67-70
205 - Gaganjeet BHULLAR (IND) 70-70-65, Boonchu RUANGKIT (THA) 70-68-67, Zaw MOE (MYN) 70-67-68, SIDDIKUR (BAN) 69-67-69, HU Mu (CHN) 66-69-70.
206 - Ernie ELS (RSA) 69-71-66, Thitiphun CHUAYPRAKONG (THA) 68-70-68, BAEK Seuk-hyun (KOR) 69-69-68, Jason KNUTZON (USA) 67-69-70, Pawin INGKHAPRADIT (THA) 67-68-71.
207 - Wade ORMSBY (AUS) 70-70-67, Jaakko MAKITALO (FIN) 72-66-69, Jake HIGGINBOTTOM (AUS) 70-68-69, Mithun PERERA (SRI) 70-66-71, Bryce EASTON (RSA) 68-67-72, Jonathan MOORE (USA) 66-68-73