What Quran says about Kudüs(Jerusalem) is it holy place for Muslims or for Jews or for Christians? Why Jerusalem is so important to everyone? I never found a good answer,what do you think,they say "IThere are three gates to hell in the world,one is in the ocean,second one is in desert,third one is in Jerusalem" is that true?

The importance of al-Quds (Jerusalem) comes from the fact that it was the first qibla (direction of prayer) to the Muslims and that it is the site of the Prophet’s ascension to heaven in his Night Journey. The Quran says:

Glory to Him who journeyed His servant by night, from the Sacred Mosque, to the Farthest Mosque, whose precincts We have blessed, in order to show him of Our wonders. He is the Listener, the Beholder. (The Quran, verse 17:1)

Above the Farthest Mosque refers to al-Masjid al-Aqsa in Jerusalem.

Jerusalem is also home to the Dome of the Rock, which is a shrine built in 691 CE on the rock from where the Prophet’s journey started. It is built on the site of Solomon’s ancient Temple. Its significance comes from its ancient history and the fact that it is one of the most recognizable monuments of Islam.

I have heard no authentic narration on there being a gate of Hell in Jerusalem.