The three southern provinces of Surat Thani, Krabi and Phangnga will start accepting people flying in from abroad under the sandbox formula on Tuesday.
The date was confirmed in the announcement of the Royal Gazette published on Saturday.
It said travellers can enter Koh Tao, Koh Phangan and Koh Samui in Surat Thani and the entire provinces of Krabi and Phangnga from Tuesday onwards.
Visitors have to follow the same protocol as the sandbox in Phuket, which requires one RT-PCR test upon their arrival at registered hotels and another five or six days later.
The Centre for Covid-19 Situation Administration on Friday added the three provinces to the sandbox model to keep the ailing tourism sector afloat after it put the quarantine-free programme, Test & Go, on ice for the forseeable future.
The final day for travellers with previously confirmed certificates to enter the country under the Test & Go scheme will be next Saturday.
Thailand has logged 1,855 imported cases so far this month, 1,166 of them through Test & Go and 538 through the sandbox in Phuket, according to CCSA figures to Saturday. They included both Thai and foreign nationals.
New infections imported through Test & Go dropped from 230 on Friday to 181 on Saturday, but the Phuket sandbox saw an uptrend of 92 to 153 during the same period.
Travellers starting their trips in the United States led imported infections since the beginning of the year with 31 cases, one more than Russia.
Tourism operators in Bangkok and Pattaya have cried foul after they were left out of the list of sandbox destinations, but the tourism promotion association of Koh Samui said on Saturday operators were looking forward to foreign arrivals when the airport gates open on Tuesday.