Nys's family claims that the law was broken because she was never treated for autism and so it had not been established that she was suffering unbearably and incurably – the key criteria for a mercy killing to go ahead.
The three doctors have been charged with failing to comply with the legal conditions for euthanasia and poisoning. If found guilty they are almost certain to face jail.
The criminal case in Ghent is the first since euthanasia was decriminalised in Belgium in 2002.
Nys, from Sint-Niklaas in Flanders, had last undergone treatment for psychiatric problems 15 years before her death.
In a television interview, her sisters, Lotte and Sophie, said she had "outgrown that" and was simply feeling rejected following the failure of a relationship.
Ghent prosecutors acted just days after a separate case in Holland. Dutch prosecutors pressed charges against a doctor in The Hague who slipped a sedative into the coffee of a 74-year-old woman suffering from dementia, then injected her with deadly drugs as her family pinned her down because she resisted.
There is deepening unease in both countries over liberal euthanasia laws that have seen rising numbers of doctor-assisted deaths. Alcoholics, sex-abuse victims, dementia sufferers and even children qualify for the right to die.
Crossbench peer Lord Carlile, co-chairman of Living And Dying Well, a UK parliamentary group opposed to euthanasia, said the actions of the Ghent authorities indicated that the euthanasia experiment was flawed.
He said: "What happens in Belgium has been held up by some as an example of good practice.
"This case calls into question all of those claims."
Dr Wim Distelmans, a doctor who campaigns for euthanasia and also chairs Belgium's regulatory commission, has since defended the three unnamed doctors facing trial.
He told Belgian public broadcaster VRT that one of the doctors might need to justify his actions, but added: "The two others only gave advice that is not binding."
Belgium's pioneering of euthanasia has prompted shock internationally, most recently by its admission that two children – aged nine and 11 – have been killed by doctors in the past two years. They were the first to die by lethal injection since Belgium changed the law to allow child euthanasia in 2014.
Figures also reveal a 13 per cent rise in euthanasia deaths over the past year and the doubling of such deaths since 2011.
Crossbench peer Lord Alton of Liverpool said any abuse of euthanasia law in Europe carried a strong message to politicians here.
He said: "These are harbingers of what could come our way. This is why Parliament has repeatedly rejected a change in British law."
