The Three Principles refer to nationalism, democracy, and people's livelihood.
Tips
history
Formulated by 孙中山 (Sun Yat-sen) in the late 1890s and developed through lectures in 1924. The three are 民族 (nationalism), 民权 (democracy / civil rights), and 民生 (people's livelihood).
usage
主义 is the suffix '-ism' or 'doctrine', as in 社会主义 (socialism). The 三 'three' counts the principles, each of which begins with 民 'people'.