By Agence France-Presse
Lum Zawng, Nang Pu and Zau Jet had helped organise an anti-war demonstration in April in the state capital Myitkyina to highlight the plight of thousands displaced by fighting between the military and ethnic Kachin insurgents.
The youth leaders were sentenced on Friday to six months in prison and fined about $320 each.
Kachin is in the grip of one of the world's longest-running civil wars, as rebels have clashed with the powerful military for six decades over autonomy, ethnic identity, drugs, jade and other natural resources in the northeast.
Fighting surged dramatically this year, forcing thousands to flee to camps in remote parts of the state with inadequate access to aid.
In late April, amid public anger over the fresh violence, the three youth leaders organised a peaceful protest demanding help for the people taking refuge in camps.
It also sparked protests in Yangon and Mandalay in a rare show of solidarity.
The defence told the court that the leaders had "no intention" of defaming the Tatmadaw, as the military is called in Myanmar, and were more concerned with the fate of the displaced people, lawyer Doi Bu told AFP.
"But the court viewed them as harming the Tatmadaw," she said, adding that they plan to appeal on Monday.
The European Union said it "deeply regrets" the court's decision and called for the sentences to be reviewed.
Three other activists protested the verdict by marching in front of the court, prompting police to charge them for "protesting without permission".
They were bailed and will face trial at a later date.
"These jail sentences reflect a pattern of continued attacks" against civil society speaking out against the military, said Amnesty International's Tirana Hassan.
"It sends a chilling warning to (anyone) who wants to tell the truth about the Myanmar military's brutality in Kachin and northern Shan states."
Rights groups estimate more than 106,000 people languish in displacement camps across conflict-torn Kachin and Shan states.
Ethnic Kachin are mainly Christians in a nation that is overwhelmingly Buddhist.
While international focus has been on the Rohingya crisis in Rakhine state, an army unit accused of atrocities against the Muslim minority group has been redeployed to Kachin -- which experts say is an ominous sign for civilians.