The Pacific Freedom Forum said Setita Tu'ionetoa, Salamo Fulivai and Vilisoni Tu'iniua were suspended last week over allegations they attempted to incite distrust in the government.
The allegations stem from comments made last year on the show Tonga Today.
Forum co-chair Ofani Eremae said the suspensions would dissuade journalists from questioning the government.
"The message that is being sent to the workers or the journalists at Tonga Broadcasting is that 'if you say something or do something that seems to be against the Tongan government you're going to get suspended or you're going to get sacked'."
The suspensions follow the November sacking of the chair of the Shared Communication Boards, Piveni Piukala.
A government statement said the grounds for termination included incompetence and failure to follow due process in appointing the chief executive of the Tonga Broadcasting Commission (TBC).
The TBC reported the sacking followed Mr Piukala's decision to decline a request from the prime minister to suspend the three staff members and to investigate them for misconduct.
Mr Eremae said the forum was calling for "transparency" over the suspensions which had added to the ongoing history of "intimidation" at the broadcaster.
"We want to remind the Tongan and Pacific governments that they should allow free speech to be exercised on public broadcasters, and that the public broadcasters must serve the public interest."