If you're only talking about East Asians (Chinese, Japanese, etc) then yes. If you are also considering South Asians (like Indians) then I'm not so sure.
all those three things are part of Chinese culture. id say that they re only the most common because threes so many Chinese people and Chinese colonies.
"In spite of the influence of Daoism and Buddhism, Confucian ethics have had the strongest influence on the moral fabric of Chinese society. "
http://www.answers.com/topic/confucianism
2) Confucianism is mostly limited to China.
3) Standard Mandarin, or Standard Chinese, known by various names to native speakers, is the official modern Chinese spoken language used in mainland China and Taiwan, and is one of the four official languages of Singapore.
I dint know if Buddhism is the most common religions in Asia, but if you are talking about the most common language, mandarin, it is right only because there are 1.3 billions Chinese in Asia.
Besides mainland china, mandarin is understood and partly used in the regions such as Singapore, Taiwan, Hmong Hmong and Macao.