Thaworn, the 2005 Asian Tour number one, was four shots ahead on another tough day at blustery Santiburi Samui Country Club before his game unraveled with some misjudged tee-shots for a one-over-par 72 and three-day total of four-under-par 209.
Varut, 23, produced a superb 66 to charge into the joint lead after starting the day six back while Siddikur, chasing a second win in Asia, rammed in a seven-foot birdie putt at the closing hole for a 71.
Pint-size Tabuena, who has held a share of the lead since day one, overcame a double bogey and two bogeys with three birdies to ensure he has a chance of becoming the fourth youngest winner on the Asian Tour.
At the end, Thaworn, who is chasing a record-equaling 13th Tour victory, was kicking himself for losing his grip on the US$300,000 Queen's Cup, the 11th leg of this season's Asian Tour.
"I only have myself to blame," lamented the 45-year-old, who won at Santiburi Samui in 2008 and was runner-up two years ago. "I didn't play well on the last three holes. Couldn't tee off well and when you get into bushes or jungle, it is not good. The double on 16 was also due to a three putt. I tried my best. You need to tee off well here to have a good score."
The precocious 17-year-old Tabuena has a chance to atone for his near miss at the ICTSI Philippine Open in February where he was in contention but stumbled with a closing 81. After a double bogey on 15, he drained a 20-foot birdie at the next hole to tie for the lead.
"I told myself to finish strong," said Tabuena, who won two local events back home recently but is chasing a first Asian Tour title. "Hopefully I can use what I learned from it (the Philippine Open) and put it into play here. Just stay patient and keep my routine and not mind the people around me.
"Anything can happen on this course. You just have to hit fairways and greens which is my game plan."
The Filipino will be 100% focused on his task ahead as he does not have any school work to think about. "I finished my third year high school (recently). Usually I bring a lot of school work on Tour but not this week because I've finished all my work," he smiled.
With good memories spurring Siddikur on as his first career top-10 was achieved in Samui before going on to secure a breakthrough victory in Brunei in 2010, the Bangladeshi converted two crucial birdies on the 13th and 18th holes.
"I saw Thaworn and the other guys missed their shots. I thought I had a good opportunity to get a birdie on the last. I'll just play my own game and don't need to do anything exceptional. Since the first day, I'm confident with my game," said Siddikur, who is currently eighth on the Order of Merit.
The surprise of the day came from 23-year-old Varut, who has never posted a top-10 on the Asian Tour. He stole the thunder with a seven-birdie round against a double bogey to put himself in position for glory.
"This is exciting, this is my big experience. Don't be afraid. Just go out and play. I'm not afraid and I've got nothing to lose," said Varut.
"My putting was very good, only 25 putts. Everything was perfect. The past few years, I learned about the game by playing on the Asian Tour. I need more practice and I know what to do and have to do."