While all three were important challenges facing newly decolonized African nations in 1960, sovereignty and security was the biggest problem in a world dominating by a Cold War between western capitalist democracies and Soviet socialism. African nations needed to establish security and sovereignty before they could address the other two challenges, and were hard put to do so in the international climate of that era.
The African nations granted autonomy in the 1950s-1960 in general faced challenges establishing sovereignty and security independent of their former colonial masters because of the legacy of colonialism. These new nations did not have good infrastructure in place when they achieved freedom: they lacked the kind of roads, power grids and communication networks common in countries with a long history of independence. Further, the colonial administrators installed by the foreign powers tended to have the most expertise in running these countries, and they left after independence, leaving less experienced administrators in charge. Finally, the political systems put in place by the colonial powers had been structured largely to benefit the people at top, not the citizens of the country as a whole. In particular, the first two issues (lack of infrastructure and expertise) often forced the African nations to rely on their former colonial masters, and the lack of a cohesive system of government that benefitted all citizens made it easy for outside governments to foment dissent.
While many African nations hoped to sidestep the politics of the Cold War powers, security concerns in both the East and the West meant that countries like the U.S. were prone to interfere in issues of African national sovereignty when they believed their own interests were at stake. For example, in 1960, Patrice Lumumba, the Congo's first prime minister, gave a speech interpreted by the U.S. and its western allies as dangerous, so the U.S., Belgium and others worked to have Lumamba assassinated and a more pro-Western government imposed, regardless of whether or not this benefitted the Congo or its people.
Thus, international political concerns of world powers during the Cold War and lack of a developed infrastructure, lack of an experienced native political class and lack of inclusive government left African sovereignty and security shaky and at risk in 1960. Problems of sovereignty and security needed to be solved before the African nations could address other issues such as basic human services and building national unity in territories with arbitrary borders established by colonial powers.