According to the Wikipedia article on the Third World, South Africa which can be found in Africa is a Third World country since the First World is defined by "The United States and its allies" a.k.a. democratic and/or capitalistic countries and the Second World by the Soviet Union, China and their allies a.k.a. soviet and/or communist countries.
The neutral and non-aligned countries such as the ones found in middle and south America, Southeast Asia and as said before Africa therefore are Third World countries.
Such being the case ... Your sentence is absolutely valid.
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