"Their detention by Spain is an affront to human rights, designed to prevent them from performing their role as political representatives of the Catalan people," lawyer Ben Emmerson told reporters.
The three include deposed Catalan vice-president Oriol Junqueras, as well as the leaders of two grassroots separatist groups: Jordi Sanchez of the ANC and Jordi Cuixart of Omnium Cultural.
Sanchez and Cuixart are being investigated for sedition. Junqueras is also in custody pending a probe into his role in the region's independence drive.
"This case does not ask the UN to adjudicate on the issue of Catalan independence but seeks the UN's reaffirmation that governments cannot repress political dissent through arbitrary detention," Emmerson said.
"Spain must release these men."
Junqueras was instrumental in Catalonia's attempt to break away from Spain via a referendum that took place on October 1st, despite a court ban, and a unilateral declaration of independence later that month.
Madrid then sacked the regional government including Junqueras, imposed direct rule on the semi-autonomous region, dissolved its parliament and called snap Catalan elections, which saw separatist parties retain their parliamentary majority in December.
Emmerson claimed that the imprisonment contravened international law and said the charges were "purely political.
"The actions of the Spanish state set a dangerous precedent... around the world," adding that they "belong to a bygone era in Spanish politics that we thought had longtime vanished".
If the decision is favourable, the lawyer expects the United Nations "to put all the necessary pressure on Spain" to release the men.
The UN panel is made up of independent human rights lawyers and has addressed hundreds of cases including prominent figures such as WikiLeaks founder Julian Assange and Myanmar's Aung San Suu Kyi.
The little-known group hit the headlines in 2016 when it found that Assange was being "arbitrarily detained" at Ecuador's embassy in London, where he fled and sought asylum to avoid arrest.
It is expected to deliver its Catalonia findings within "weeks or months, rather than years", according to Emmerson.
The working group's opinions are not considered binding and states can demand a review of the decision within two months if they supply new information that challenges the basis of the decision.