"I regret this act and I apologise," Josephine Markmann, the German member of the group, told the judge, who said Muslim law prohibited such behaviour.
"We didn't expect to shock Tunisians to this extent. It is out of the question that we would do it again," said Pauline Hillier, one of the two French women also appealing a four-month prison sentence for their demonstration last month in support of a fellow Tunisian activist.
The head of the Femen movement in Paris, Inna Shevchenko, admitted the apologies were an "unexpected turnaround of the situation," but said she had "suspicions that the activists have suffered enormous psychological pressures."
The three women had appeared at the court of appeal in Tunis dressed in the traditional Tunisian veil, or safsari.
Also Read
Lawyers for a number of Islamist groups angered by the protest, who are seeking to participate in the trial as a civil party, criticised the short time it took to organise the appeal hearing and asked for another delay, which the judge refused.
But French defence lawyer Patrick Klugman accused the Islamists of trying to prevent the case from ever being resolved, and argued that there was nothing sexual about the women's protest.
"You cannot pervert the message of Femen. Their breasts were visible to the public but they were carrying a message you can't ignore. Stop looking at their breasts... And listen to them," Klugman told the court.
He added that the women believed they would be safe staging their protest "in a country that has just risen up for freedom."
They were jailed for four months on June 12 for indecency and an attack on public morals, sentences criticised as harsh by international rights groups and European countries.