Three African Presidents met in secret session in Lusaka on Wednesday amid intense speculation of a new initiative to achieve a settlement of the Rhodesian dispute.

The mystery has been deepened by reports that the three heads of state - Zambia's Kenneth Kaunda, Tanzania's Julius Nyerere and Sir Seretse Khama of Botswana -- had been joined in their talks by two Rhodesian nationalist leaders, temporarily released from prison by the Rhodesia Government for a second time to attend the talks.

If the reports are correct, it's the second time in a month that the three Presidents have met with Mr. Joshua Nkomo of the Zimbabwe African People's Union and the Rev. Ndabaningi Sithole of the Zimbabwe African National Union.

The Lusaka summit has been given added impetus by the South African policy of trying to persuade Rhodesia to seek a settlement in the dispute, which began nine years ago when Rhodesia's white minority declared independence from Britain.

The diplomatic activity in Lusaka was increased by the arrival yesterday of Dr. Jonas Savimbi, leader of the Angolan UNITA movement, who flew in for consultations with President Nyerere before continuing his journey to Dar es Salaam.