Two of those allies, Marshall Islands and Palau, have long-standing defence and funding compacts with the United States, while a third, Tuvalu, entered a security and migration agreement with Australia in November.
With Nauru's switch, Taiwan's allies in the region have halved from six to three since 2019, amid increased US-China rivalry for security ties and offers of infrastructure and funding.
The remaining 15 Pacific Island Forum regional bloc countries have diplomatic ties with China.
China claims Taiwan as its territory with no right to state-to-state ties, a position Taiwan strongly disputes.
Worldwide, a dozen countries maintain formal diplomatic ties with Taipei.
Taiwan will send a special envoy to Marshall Islands President Hilda Heine's inauguration. (AP PHOTO)
In a statement on Thursday, Marshall Islands said its ties with Taiwan were "rock solid" and in the region's best interest.
"The Republic of the Marshall Islands values the strong relationship with Republic of China (Taiwan) as an indispensable partner in promotion of democratic principles," a statement on its official Facebook page said.
"In parallel, we wholeheartedly respect the sovereignty of all countries and will continue to foster open and friendly dialogue with other nations for the sake of peace and stability for all."
Taiwan's foreign ministry said on Wednesday that Deputy Minister of Foreign Affairs Tien Chung-kwang would attend the inauguration of Marshall Islands President Hilda Heine on Monday as a special envoy of President Tsai Ing-wen.
Taiwan also said Tuvalu Prime Minister Kausea Natano had reaffirmed his country's ties with Taiwan in a message to its embassy in Funafuti.
Tuvalu's government, in a caretaker period ahead of a national election on January 26, congratulated Taiwan's new president-elect, Lai Ching-te, on its Facebook page.
Palau wrote on Facebook it stands with Taiwan, which it called a "good friend", adding that nations were free to choose who they could build diplomatic ties with "without coercion".
Palau's President Surangel Whipps had written to Lai, saying his nation wanted to "further strengthen our already rock solid alliance at the soonest opportunity", Whipps' office said.
