The workers, Somphone Phimmasone, 30, Soukan Chaithad, 33, and Ms Lodkham Thammavong, were sentenced in early April to prison terms of between 12 and 20 years.
A harsh message on human rights
The International Federation for Human Rights (FIDH) and the Lao Movement for Human Rights (LMHR) said the sentences sent a "chilling message across Lao civil society that the government is determined to crush the slightest sign of activism and opposition to its authoritarian rule."
While in Thailand, the migrant workers posted messages on social media critical of the government, alleging corruption, deforestation, and human rights violations.
They also participated in a protest against the government outside the Lao Embassy in December, 2015.
They were arrested in March 2016 after returning to Laos to reapply for official documents before planning a return to Thailand.
Government accuses the 3 of 'threatening national security'
Lao state-run television showed Somphone, Soukan, and Lodkham, being held in custody at the police headquarters in Vientiane. Official reports accused the three of threatening national security and tarnishing the government's reputation.
Andrea Giorgetta, FIDH Asia Desk director, said the arrests highlighted the government's close monitoring of citizens abroad.
"The government of Laos went out of its way to persecute these three dissidents actually based in Thailand. It shows that the government is also stepping up on-line monitoring of its citizens because these three have expressed their opinions and criticisms of the government policy," Giorgetta told VOA.
Laos classified as 'not free'
The U.S.-based non-governmental organization Freedom House, in its assessment of the civil liberties and media rights, classifies Laos as 'not free', with low or zero ratings on political right and liberties.
In 2016, Freedom House noted Lao authorities were increasingly attentive to criticism on social media, detaining citizens for "contentious posts" ahead of Laos chairing meetings of the Association of South East Asian Nations (ASEAN).
Phil Robertson, deputy Asia director for Human Rights Watch, said the migrant workers had taken "advantage of the relative freedom" they experienced in Thailand to criticize the Lao authorities.
"The criticisms should not be a crime," he said, adding the three were held for several months in pre-trial detention. The sentencing highlighted the "shortcomings in the Lao judicial system," he said. "There is a complete lack of transparency and accountability within the Lao judicial system, which you see when people don't have access to lawyers, trials are conducted in secret, families are only informed well afterward of proceedings against their loved ones."
No tolerance for criticism
The verdicts add to a list of arrests and forced disappearances of activists and protesters who have been critical of issues ranging from land disputes to allegations of corruption and abuse of power.
The high profile disappearance in December 2012 of well known civil society leader Sombath Somphone, after he was seen being arrested at a police checkpoint, remains unresolved amid calls for transparency in the case.
Shalmali Guttal, a spokesperson for "The Sombath Initiative", said harsh sentencing by authorities in Lao has been on-going over several years as regional governments also look to tighten controls over social media.
A long history
"This is a trend in Lao for sure. It's been going for a very long time of course because there is no critical discussion publicly about policy, about governance, about how the affairs of the state and society is conducted. So yes, that's been going on. It is also part of this trend in the region," Guttal said.
Other cases include the 2009 detention of a group of men and women planning to participate in pro-democracy demonstrations in Laos, while in 2007 an outspoken critic of Chinese sponsored agricultural projects also disappeared.
FIDH's Giorgetta said with the existing media outlets tightly controlled, increasingly people and Lao civil society have turned to social media to express grievances.
"We have seen arbitrary arrests of activists who have exposed cases of corruption and bad governance," he said.
Huge construction projects underway
Of key concern are major infrastructure projects, especially by Chinese and Vietnamese investors, including the China-led $6.0 billion, 415 kilometer rail line from northern Laos to the capital Vientiane.
"The infrastructure and development projects being implemented in Laos - but that merely results in massive human rights violations - like the case of the Lao China railway that just started [in construction]," he said.
Robertson's Human Rights Watch says a major concern for the three migrant workers will be to survive the harsh prison conditions.
He said the Lao People's Revolutionary Party has applied its "full force to basically ruin these people's lives and throw them behind bars for long sentences, which given the very poor conditions in Lao prisons, for some of them could be a death sentence."