The move follows espionage charges laid earlier this month against six reporters from the independent daily newspaper.
The three other journalists, Douglas Hampande, MacPherson Muyumba and Reuben Phiri, were charged on Saturday and told to appear in court in Lusaka with the six others on 16 April, their lawyer, Sakwiba Sikota, said. They appeared briefly in court and were not detained. "We expect more arrests to come," Mr Sikota said.
Six Post journalists were granted bail on Thursday on accusations of exposing "state security information to the enemy". If convicted, all nine journalists face minimum 20-year prison terms and maximum terms of life in prison.
On 9 March The Post quoted unnamed senior military officers criticising Zambia's ability to defend itself, saying it would not withstand any attacks by the far superior army of neighbouring Angola. Relations between the two countries have been tense since Angolan alleged that Zambia is supplying arms to Angolan rebels, a charge Lusaka denies. (AP)