Hachiman Jinja are the second most numerous in Japan, after Inari Jinja.

According to Wikipedia, 八幡　was originally  read as Yahata, i.e. the pure Shinto reading,  while the reading Hachiman derives from Buddhism.

Hachiman-Kami is popularly thought of as being the God of War, although he is formally the God of Warriors. He is the divine spirit of Japan’s 15th emperor Ōjin (応神天皇) and is often enshrined under his given name, Homuda-wake-no-mikoto  (品陀和氣命). His mother, Empress Jingū  (神功皇后) is often enshrined along with him, as is another female Kami,. Himegami (比売神).

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​ In 571 the 29th emperor,  Kinmei (欽明天皇), formally declared that the divine spirit of Emperor Ojin was first revealed in what is now the city of Usa, (宇佐), in Oita Prefecture. He became the tutelary Kami of the city, along with his mother,  Empress Jingū  (神功皇后), and another female Kami, Himegami (比売神). All three, known together as the Three Hachiman  Kami (八幡三神), are now enshrined in Usa Jingu.

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However, the Three Hachiman Kami are not enshrined at all Hachiman Jinja. While Emperor Ojin/ Homuda-wake-no-Mikoto is enshrined at virtually every Hachiman Jinja, and is often the sole enshrined Kami, Empress Jingu is sometimes replaced by his father, Emperor Chuai, and Himegami by Tamayori-hime (玉依毘売命). Takenouchi no Sukune (武内宿禰), who is said to have served under five emperors, including Chuai and Ojin, and to have accompanied Empress Jingu on her invasion of Korea, is also enshrined at some Hachiman Jinja e.g. Tomioka Hachiman-Gu

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The head shrine of the Hachiman group is Usa Jingu and there are three other shirines considered very imortant to the group.

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Usa Jingu                                 Oita-ken, Usa-shi, Minami-usa 2859

Iwashimizu Hachiman-Gu  Kyoto-fu, Yawata-shi, Takabo 30

The Three Hachiman Kami are enshrined here

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Hakozaki-Gu                          Fukuoka-ken, Fukuoka-shi, Higashi-ku
                                                   Hakozaki 1-22−1

Emperor Ojin and Empress Jingu are enshrined here; Himegami is replaced by Tamayori-hime

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Tsurgaoka Hachiman-Gu    Kanagawa-ken, Kamakura-shi,                                                                 Yukinoshita 2-1-31

The Three Hachiman Kami are enshrined here