Registration in Omaha began Tuesday, and expatriates are turning out in 'droves' to register, reports said.
Most observers expect the South to opt for independence, an outcome even the United States has labeled "inevitable", AP reported.
China has also endorsed the referendum, calling for it to be 'peaceful and transparent'.
The United Nations Security Council on Nov 16 reaffirmed its "strong commitment to the sovereignty, independence, peace and stability of Sudan" – while at the same time welcoming the start of registration for the referendum and encouraging "further efforts to ensure" that it takes places.
The three sites in US are in Omaha, with its large population of Sudanese refugees, Washington D.C., and Phoenix. People will need to register ahead of the Jan. 9 vote, then return to cast their ballots.
The referendum is part of a 2005 peace agreement between Sudan's government and the South's Sudan People's Liberation Movement (SPLM) that ended a 21-year civil war.
The deal included the referendum on independence for southern Sudan in January 2011 and a separate self-determination vote for Abyei, a disputed border zone.