Israeli Prime Minister Menachem Begin promised a group of settlers that the government would begin building a Jewish town at Givon, in the occupied West Bank, within three months, the settlers said. Some 300 Israelis squatted on the site for two days, saying the government had broken its promise to begin building a Jewish settlement there. Tuesday night they left the hilltop after an appeal by Mr. Begin. But Wednesday they emerged from a meeting in which the prime minist er assured them a town would be erected.