UPDATE 1-Thai police make third arrest in stock rumour probe

* Snooker club owner admits to posting web messages

* Suspects accused of spreading rumours on Internet

BANGKOK, Nov 4 (Reuters) - Police in Thailand said on Wednesday they had arrested a third person on suspicion of spreading rumours about the health of the country's 81-year-old king that triggered a slide in stock prices.

Somjet Itthiworakul, a 38-year-old snooker club owner who also organises trips to border casinos, was arrested late on Tuesday in Chon Buri province, 100 km (62 miles) east of Bangkok, after posting comments on Thai website Prachatai.

"He admitted he had posted messages on the website," said Police Commander Tha-ngai Prasajaksatru, head of Thailand's Central Investigation Bureau.

"We've found that there is information linking the suspect to the two previously arrested. They've been posting messages back and forth on that website," he said.

Thai stocks .SETI plunged 7.2 percent on Oct. 14-15 as rumours circulated over the health of the king, who has been in hospital since Sept. 19. The palace says he is getting better and he appeared in public in the hospital grounds on Monday. [ID:nBKK495082]

King Bhumibol Adulyadej, the world's longest-reigning monarch, is regarded as semi-divine by many of the country's 67 million people.

His health is a sensitive topic in financial markets because he is seen as the sole unifying figure in a politically polarised country with a long history of coups and upheaval.