Dawn Blalock said no violence was reported during the abduction in the northwestern Somali town of Wajid. There had been no contact with the kidnappers so far.
"The United Nations and the local authorities ... are doing everything to secure the unconditional release of the staff," she said in a statement.
A witness said a Somali translator was later released. Blalock could not confirm the translator´s release or the nationalities of those abducted, but said earlier reports that two of the abductees were a Russian and a Ugandan were incorrect.
The witness said the three had only been in Wajid for one night and were on their way to the town´s airstrip when they were seized. The witness asked for anonymity for fear of retaliation by the kidnappers.
According to the UN, a total of 35 aid workers were killed in Somalia in 2008 and 26 were abducted. Two aid workers have been killed this year already.
The arid Horn of Africa nation had not had a functioning government since 1991 and is riven between clan warlords and an Islamist insurgency battling the weak UN-backed government. Nearly half its population is dependent on aid.
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AP writers Katharine Houreld and Malkhadir M. Muhumed in Nairobi, Kenya, contributed to this story.
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