They were responding to a blockade set up by several dozen members of a far-right group, Generation Identitaire (Identity Generation), at a nearby pass to keep them out.
The trial is the latest involving activists trying to aid migrants on the move through Europe, in protest at a tough new immigration law being pushed by French President Emmanuel Macron.
About 120 leading French education, scientific and political figures signed a tribune in French daily Le Monde this week supporting the three activists, accusing prosecutors of flouting France's constitutional promise of fraternity and equality for all.
Several dozen protesters gathered outside the courthouse in Gap, southeast France, as the proceedings opened to stage a "reverse trial" against the government.
"We're accusing the state and the government's migrant policies," said Michel Rousseau of the Tous Migrants (We're All Migrants) association.
Lawyers for the three told AFP they would seek to postpone the hearing while awaiting a ruling from France's top Constitutional Court on whether aid to illegal immigrants should be considered a criminal offence.
They also hope to ease the terms of their conditional release from jail, so they can pass end-of-term exams or return to their jobs.
The activists risk up to 10 years in prison and fines of up to 750,000 euros ($880,000), as well as a ban on entering France.
Their trial comes a day after a 73-year-old Amnesty International volunteer appeared in court in the southern city of Nice on charges of helping two underage Africans illegally enter the country.
French lawmakers voted last month to soften laws criminalising acts of solidarity with illegal migrants, to exempt those who provide them with free food, shelter, medical care or legal advice.
Helping migrants illegally cross the border remains a crime, however.
The government argues that tighter controls are needed to check the rise of anti-immigration populists who claim Europe has allowed in too many people seeking a better life.
The law being debated in parliament aims to both cut waiting times for asylum applications -- to six months from around a year currently -- and make it easier to deport those turned down as "economic" migrants, as opposed to those fleeing political strife.
NGOs have contrasted the treatment of the campaigners with that of "Spiderman" Mamoudou Gassama, the illegal Malian migrant who was fast-tracked for French citizenship this week after rescuing a young boy hanging from a balcony in Paris.
"A black person who saves a white person deserves praise, a white person who helps a black person deserves prison," said Giscard Destin, a 23-year-old from Cameroon, at a rally to support the three activists in nearby La Roche-de-Rame on Wednesday.
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