John Harding, Andrew Hill and Dylan Healy are among five European men who appeared in a court administered by Moscow-backed separatists in the eastern Ukraine city of Donetsk.
The five - including Swede Matthias Gustafsson and Croat Vjekoslav Prebeg - all pleaded not guilty to charges of mercenarism and “undergoing training to seize power by force,” according to Russian media.
Prosecutors allege all five men were members of the Azov Battalion and other military units captured in Mariupol.
They could face the death penalty under the laws of the self-proclaimed Donetsk People’s Republic.
The court is not internationally recognised.
The men's next hearing is scheduled for October, the Interfax news agency reported, citing a statement by the separatists' court.
Mr Healy, 22, is a chef and aid worker from Cambridgeshire and Mr Hill, 35, is a military volunteer from Plymouth. Fewer details are publicly known about Mr Harding.
Allan Moore, who has been friends with Mr Healy for almost three years, previously told ITV News Anglia his football teammate had gone to Ukraine "to try and help and make a difference".
Dylan Healy, 21, from Cambridgeshire, was reportedly caught along with another British man, Paul Urey, on Monday at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine.
According to UK non-profit organisation Presidium Network, Mr Healy was trying to help people flee areas of the country under Kremlin occupation when he and another British aid worker, Paul Urey who later died in Russian captivity, were captured at a military checkpoint in south-eastern Ukraine.
It comes after a video shown on Russian television in April featured a man speaking with an English accent who appeared to give his name as Andrew Hill from Plymouth, saying that he had four children.
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In June, the unrecognised Donetsk court sentenced British men Aiden Aslin, 28, from Nottinghamshire, and Shaun Pinner, 48, from Bedfordshire, along with a Moroccan man, to death for being mercenaries.
The Foreign Office slammed them as “sham judgements” and the European Court of Human Rights forced to intervene in the case.
All three, who were captured by pro-Russian forces in Ukraine’s industrial east, have appealed their verdicts.
Speculation is swirling that the Kremlin may seek to use the foreign fighters to extract concessions from Ukraine or swap them for Russian prisoners.