It was the 76th day without domestic cases since May.
The three new cases were all asymptomatic and found in Bangkok state quarantine hotels. A male Bangladeshi merchant, 31, a 41-year-old Indian woman, and a 30-year-old female Thai hotel employee who had returned from the United States all tested positive.
Meanwhile, one patient was discharged after recovery.
As of August 9, the total number of confirmed cases in the country stood at 3,351 (414 in state quarantine) -- 142 are under treatment, 3,151 have recovered and been discharged, and there have been 58 deaths.
Globally, the total number of confirmed cases reached 19.8 million, up by 261,000 on Saturday, 12.7 million have recovered while deaths stood at 729,000. Thailand is 112th among countries with most number of cases. The US has the highest figure (5.1 million), followed by Brazil (3 million) and India ( 2.1 million).