After leading on the first two days with rounds of 66 and 64, Guan remained at 14-under, but the big-hitting Goss, a quarter-finalist at this year's US Amateur, closed in on his playing partner with a bogey-free 69 at Amata Spring Country Club in Chonburi.
The winner earns an invitation to next year's Masters Tournament, while the champion and runner(s)-up earn spots in International Final Qualifying (IFQ) for The Open Championship.
Thailand's Prin Sirisommai, who also played in the final group, carded a 72 to stay nine-under, one ahead of Korean Amateur champion Lee Soo-min, last year's runner-up, who signed for a 69.
Chinese Taipei star Pan Cheng-tsung, Asia's No. 2 amateur, shot 67 to move to seven-under, one ahead of defending champion Hideki Matsuyama (70), Asia's No. 1, and his Japanese compatriot Mikumu Horikawa (67).
Guan "“ sporting lime-green check trousers "“ started five ahead of Goss and Prin, but dropped back with bogeys on five and nine during a tension-filled front nine.
However, the Guangzhou-based schoolboy settled down on the back nine and picked up birdies on 11, 12 and 14, before bogeying the 18th for the second time in three days.
"We all started a little bit slow. I felt a bit nervous and everybody looked a little bit nervous to start with, and there was not much talking. I then started focus on my own game and felt pretty relaxed on the back nine, got a few birdies, so it was all right," said Guan, winner of last year's 11-12 division in the Junior World Golf Championships in San Diego.
"It was a different atmosphere because on the first two days I knew who I played with and we talked much more. Also, today, the pin locations were a little bit harder and a couple of the tees were set back, so it was a little bit more difficult for me."
The slender Guan, who hits the ball about 250 yards with his driver, is seeking to become the event's first Chinese winner, but will have to hold off the powerful and more experienced Goss, who won last week's Western Australia Open to add to his Western Australia State Amateur victory in March.
"Goss hits the driver pretty far and every part of his game is pretty good. He doesn't make big mistakes. He's a very good player," said "˜Langlang', who trains in California for three months each summer.
The competitive Goss is determined to become the first Australian to win the championship and is expected to mount a strong charge on Sunday as he bids for an invitation to next year's Masters Tournament at Augusta National Golf Club.
"I'm still in the hunt. I was three-under today, so I'm not disappointed, although I didn't really give myself many opportunities and didn't hole many putts," said the Perth-based teenager, who heads to the University of Tennessee next January.
"Guan did well because the course was playing a lot tougher than the first two days. He doesn't hit it as far as other players, so I think he did really well. I hit it longer than him, but I think he's too young to be intimidated."
Matsuyama showed his competitive spirit when he admitted he had not given up his bid for a third straight title, despite being eight shots off the pace.
"I really want to win, so I'll do my best. I just want to win. I know Guan is very good, but I need to beat him," said Matsuyama, who has made the cut on his two appearances at the Masters Tournament both earned through back-to-back wins in the Asia-Pacific Amateur Championship.
Organised by the Asia Pacific Golf Confederation (APGC), the Masters Tournament and The R&A, the championship is the world's most televised amateur golf tournament, aired in more than 150 countries, and features two hours of live broadcast on each of the four days and a 30-minute post-event highlights show.