Jerusalem is your answer
Jews: Jerusalem was the capitol of ancient Israel, and was also home to the ruins of the Temple built by King Solomon during the Golden Age of Israel.
Christians: Christians typically trace their roots back to Judaism, as they also believe in the old testement. In fact, the church was started in the area of Judah, which was close to the original Temple site.
Muslims: Muslims believed that Jerusalem was the home to the prophets of Islam, and the Qur'an also mentions Jerusalem many times, making the city important.
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