Data analysis: The source of Sanjiang River is located in Qinghai Province, which is called China Water Tower, and it is the most important water conservation area in the upper reaches of Yellow River, Yangtze River and Lancang River. (1) The source of the Three Rivers, known as the "China Water Tower", is located in the Qinghai-Tibet region of China.

(2) The importance of establishing "Sanjiangyuan Nature Reserve" lies in protecting and improving the ecological environment, protecting rare and endangered animals, and making the economy and society of the upper, middle and lower reaches sustainable;

(3) Sanjiangyuan Nature Reserve mainly protects the endangered animal Tibetan antelope.

So the answer is:

( 1)A；

(2)D；

(3) answer.