The trio is charged with violating royal defamation laws in addition to sedition charges slapped on all 18 activists indicted on Monday over a rally in central Bangkok in September.
Three prominent Thai pro-democracy figures have been denied bail in a Bangkok court after being charged with insulting the monarchy, as the government escalates a legal row with a youth-led movement that emerged last year.
The three activists – Panusaya "Rung" Sithijirawattanakul, Panupong "Mike" Jadnok and Jatupat "Pai" Boonpattararaksa – were charged with violating royal defamation laws over a rally in central Bangkok in September, Prayut Pecharakun, a spokesperson for the attorney general, said.
They were also charged with sedition alongside 15 other pro-democracy protesters, the spokesperson added.
All three face other royal defamation charges.
At the peak of the protests, tens of thousands massed in the streets of Bangkok demanding the resignation of Prime Minister Prayut Chan-O-Cha's government and a new constitution.
The movement also broke long-held taboos by calling for reforms to the monarchy, in a country where the royal family is considered untouchable and has been treated with reverence for decades.
The monarchy is protected by the kingdom's harsh lese majeste laws – referred to as "112" by its penal code section – which carry penalties of up to 15 years per charge.
The activists marched to a Bangkok criminal court on Monday morning flanked by scores of supporters carrying flags and posters of other detained protest leaders.
'Fight on everyone'
Jatupat posted on Facebook on Monday afternoon that he, Panusaya and Panupong had been remanded in custody.
"Fight on everyone," he wrote.
Fourteen activists on sedition charges were granted bail, but another one faced legal action over a separate incident and remained in custody.
Riot police were deployed to the court compound, as dozens of protesters chanted for the release of the trio, but later dispersed.
'Will continue fighting'
Earlier Panusaya, who is facing eight other royal defamation charges, said the pro-democracy movement would continue to exist although most of the leaders could end up behind bars.
"No matter how many people are locked up, people outside will continue fighting, they do not need us," she told reporters.
"I am not concerned at all that the movement will stop."
In February, four other protest leaders were also indicted on lese majeste charges. They have been repeatedly denied bail.
They included Parit Chiwarak, also known as Penguin, and human rights lawyer Anon Numpa – arguably the two most well-known faces of the movement.
Penguin is facing 17 charges of lese majeste for his role in the protests, while Anon has been charged 11 times so far.
The continued detention of the four sparked rallies in Bangkok last month including one where police used rubber bullets, tear gas and water cannon on protesters.
At least 57 other protesters face royal defamation charges in Thailand, including three minors.