Representatives of 25 Third World nations began preparations here today for a special meeting of the nonaligned movement on southern Africa after a three-hour delay sparked by a dispute over the status of the delegation from Cambodia.

Despite a demand from the new Vietnamese-backed People's Revolutionary Council in Cambodia that it be recognized as the "sole legitimate government," the conference decided to recognize the four-man delegation from the fallen Pol Pot government.

The delegation, headed by Penn Nouth, a former premier and close associate of Prince Norodom Sihanouk, will not be allowed to take part in the discussions but the compromise was seen as a partial victory for the Pol Pot side. A number of delegations expressed privately their continuing support for it while condemning the armed intervention of Vietnam.

The issue of involvement by nonaligned nations in the affairs of other members severely disrupted the last meeting of the bloc, when foreign ministers gathered at Belgrade.

At that meeting last July, host Yugoslavia presented a resolution interpreted as Critical of the introduction of Cuban troops in Africa. The dispute ended only when the whole issue was dropped.

White the fall of the Pol Pot government early this month, Yugoslavia and other leading nonaligned nations were among those criticizing the role of Vietnam, with Cuba defending it.

Ambassadors from the 25 member nations of the Coodinating Bureau of the Nonaligned Movement now are preparing for a special three-day foreign ministers' conference here Tuesday.

The conference has been called primarily to mobilize support among the nonaligned for the liberation movements of southern Africa. Mozambique, the host, is known to be anxious to aviod the introduction of any other issues that might distract from the main topic.

The meeting is also expected to draw up resoulutions to be submitted to the conference of heads of government from the 86 nations now belonging to ghe nonaligned movement and scheduled to gather in Havana this summer.

The issue of Cambodia came up even before the meeting formally opened. There nations on the coordinating bureau -- Afghanistan, Cuba and Vietnam -- have already recognized the new government and reportedly spoke against seating the Pol Pot delegation.

There are conflicting reports of whether the new government intends to send a delegation but the Foreign Minister Hun Sen, asking the coordinating bureau "to take the necessary measures to do away with the presence of those self-styled representatives" of the Pol Pot government in the nonaligned movement.

A spokesman for the Pol Pot delegation, Chan Youran, Insisting that there was no question of the conference not allowing it to participate, denied that his delegation had any intention of introducing the question of Vietnam's invasion.

"This conference is about southern Africa and we are not going to raise the issue of Kampuchea," he tole reporters.

The Mozambican-written draft document on the southern Africa situation is sharply critical of Western involvement in the region, particularly of the British and U.S. attempt to find a negotiated settlement to the Rhodesia dispute.

Without mentioning the two Western countries by name, it accuses them of seeking to isolate the liberation movements from the nonaligned movement and socialist countries and of backing "puppet groups" in Rhodesia and Namibia.

It also accuses them of trying "to lull the oppressed masses into abandoning the [armed] struggle" in those two white-dominated nations.