創立, 創設, and 設立: objects, commemorations, and preparation
Compare the founding of a school, company, theater group, library, and reading group while separating commemorations from discussions before establishment.
創立: a school's anniversary and a company founded
創立 (そうりつ) means founding or establishment. 「学校の創立百周年を祝う式典が開かれた。」 means “A ceremony marked the school's hundredth anniversary.” The event held was a ceremony marking the school's hundredth anniversary. 創立 refers to the school's founding, while the event occurring in this sentence is its commemoration.
「三人の技術者が小さな会社を創立した。」 means “Three engineers founded a small company.” Three engineers founded a small company. Both this event and the separate school anniversary use 創立, so the word is not limited to schools or large organizations.
創設: establishing a theater company or library
創設 (そうせつ) means establishment or founding. 「若者たちは町に小さな劇団を創設した。」 means “The young people founded a small theater company in the village.” The young people founded a small theater company in the village. 創設 describes establishing the group here; it does not report a performance or its success.
「図書室の創設を記念する展示が開かれた。」 means “An exhibition was held to commemorate the establishment of the library.” An exhibition commemorated the library's establishment. The library and the theater company are different objects, and the exhibition is distinct from the act of founding the library.
設立: founding a group and discussing needed roles
設立 (せつりつ) means establishment or founding. 「住民が小さな読書会を設立した。」 means “The residents established a small reading group.” The residents established a small reading group. This example explicitly describes founding a group. The meanings of 創立, 創設, and 設立 overlap, so the words alone do not determine an organization's size or legal status.
「団体の設立に必要な役割を話し合った。」 means “We discussed the roles needed to establish the organization.” The discussion concerned roles needed to establish an organization. It reports preparatory discussion, rather than confirming that the organization has been established. Keep it separate from the example where residents actually founded a reading group.
Do a 創立百周年 ceremony, an exhibition commemorating 図書室の創設, and a discussion of roles needed for 設立 all describe founding an organization now?
Show the answer and why
No. The first two describe commemorative events, while the last describes discussion in preparation for establishment. Keep them separate from the company, theater company, and reading group that were explicitly founded in other examples. These words do not form fixed categories based on legal status or size.