Kimsong, O'Byrne and Baliga were all targeted by Sivakumar in an internal memo savaging the Post's coverage of the newspaper's sale, with the new owner calling on all three staff members to be "terminated".
Several other members of staff, including managing editor Stuart White, digital director Jodie DeJonge and business editor Brendan O'Byrne, quit in protest at Kimsong's sacking.
Staff were reportedly told that all future articles would need to be approved by a new editor-in-chief before publication.
The paper was founded in 1992 as Cambodia tried to rebuild following the genocide carried out by the Khmer Rouge in the 1970s. The Australianmentioned the paper had been "remarkably" free from authorities interference.
Last year, the leader of the largest opposition party was arrested on charges of treason and the party was later forced to dissolve.
Cambodia has elections in July.
"All the undersigned express our disgust for this decision made in contradiction to the values of a free press".
"This region is full of turbulence and the recent changes within Cambodia ahead of, leading up to, the upcoming election, has put a spotlight on the Phnom Penh Post, as the last truly independent media group in the country", the statement said.
This "cannot be concluded based on what happened between the firm and the client more than 25 years ago", the statement said.
Although a press release from the paper's new owners announcing the sale maintained that Sivakumar was "fully committed to upholding the paper's 26-year-old legacy and editorial principles/independence without infringing any relevant laws and regulations of the Kingdom of Cambodia", the memo - which was published earlier today by local news site AEC News - served up a stinging rebuke to the article, claiming that the piece did not meet the "high caliber" [sic] that the new owners expected from the paper.
Kimsong told Southeast Asia Globe magazine that he was sacked because he had "allowed the printing of an independent story based on journalistic integrity".
Huy Vannak, an Interior Ministry official, said that the prime minister was "too busy doing the affairs of the nation" to concern himself with the sale of a newspaper, adding that he was unaware of any prior business relationship between Hun Sen and Sivakumar's company. Its website says, without elaborating, that Hun Sen's "entry into the government seat" was among its government-related projects.
Crispin said he anxious that the tax bill The Post was facing might have been a pressure tactic used to force the newspaper's previous owner, Bill Clough, to sell, rather than fight the bill in court.
The Post has also been dogged by an ongoing legal action launched by former chief executive Chris Dawe for wrongful dismissal during his tenure at the paper. His personal assistant, Krishna Kumar, said Monday in an email that he was unavailable for comment and would be out of the office for about two weeks.