An engaged couple, Jung Hyun-seon and Kim Ki-Woong, and Park Ji-young, 22, were named "martyrs"  on Monday.
More than 300 people died when the Sewol ferry went down on 16 April. Many were high school students.
The captain and other crew members have been criticised for abandoning ship while passengers were still on board.
The ferry, carrying 476 people, was sailing from Incheon to Jeju Island when it went down. Only 172 people were rescued, with many others trapped inside the ship as it sank.
Park Ji-young, reportedly the youngest crew member, gave her life jacket to a passenger. She died while struggling to make sure passengers on the upper floors of the ferry wore life jackets and found their way out.
"Park pushed shocked passengers toward the exit even when the water was up to her chest," a survivor told local media after being rescued.
Jung Hyun-seon and Kim Ki-Woong were  due to marry later this year, according to local media reports.
"They were together for four years," Mr Kim's mother told Yonhap news agency earlier. "I only hope the two of them will be happy in a nice place... my heart is about to burst."
Their designation as martyrs means they will be eligible to be buried in a national cemetery, and their families can receive financial compensation and medical assistance.
Also on Monday, the Ministry of Oceans and Fisheries revoked the license of ferry owner Chonghaejin Marine Co, saying it had contributed to the disaster.
The ferry is believed to have been carrying more than three times the authorised amount of cargo.
The captain and 14 other surviving crew members have been arrested on suspicion of manslaughter and breaking maritime law.
The episode has been marked by allegations of cowardice, corruption and incompetence, BBC Asia analyst Michael Bristow reports.
But the South Korean government has now recognised that there were also at least three heroes on board the ship, he adds.
Three crew members who died saving passengers as a South Korean ferry sank last month have been honoured by the government.