Police said that the three officers were killed when a bomb planted along railway tracks in the Chana district of Songkhla province exploded early on Wednesday.
Another officer was injured in the attack, which was believed to have targeted police who conduct daily inspections of tracks before the departure of morning trains.
Wednesday's blast, which happened at about 6:30am (2300 GMT), came hours after a series of 100 co-ordinated bomb and arson attacks struck government and civilian targets across Thailand's Muslim-majority south, leaving at least three people injured.
Officials said that homemade devices, many triggered by remote-control, went off in the homes of police and government officials, karaoke bars and a train station in the southern provinces of Yala, Pattani and Narathiwat, along the Malaysian border late on Tuesday.
A rubber factory in Pattani province was set on fire in what appeared to be the most serious incident.
Thai television said the attacks began at about 8pm and included the burning of police cars and public phone booths.
It said two karaoke bars in Narathiwat province had been bombed, but no casualties were reported.
Reports from journalists in the area, based on information from police and military officials, said Molotov cocktails had been thrown at a Buddhist temple in the Rue So district of Narathiwat.
Reports said that a 5-kilogramme bomb was triggered by a mobile phone outside a local government office in the province's Cho Airong district as an army patrol passed by, slightly wounding one soldier, and assault rifles were fired at the homes of police and teachers in the province.
Major Woraphong Siewpricha, the regional deputy-chief, said that at least three people were reported hurt in the attacks.
Colonel Somkuan Kamphira, deputy-chief of the regional police investigation unit, said more than 100 attacks occurred across the area, although many involved only the burning of tyres.
The incidents were the largest and best organised attacks in more than a month in the south, where 1,400 people have died over the past two years in attacks blamed mainly on Islamic separatists.