Mecca is an important place because it is the center of the Islamic religion,it also has the great mosque- also called the Ka'ba.
It is also important because Muslims pray always toward Mecca. Mecca is the birth place of Islam.
It is also where Muhammad was born, and where he was kicked out of.
Medina is also an Important place because, for one thing, it is where Muhammad left to when he got kicked out of Mecca. Medina is also the place where Muhammad was buried underneath the green dome of a mosque.when Muhammad was alive, the first mosque built in Medina. it's called Masjid Quba'. The mosque was eventually destroyed by lightning in 850 CE.

Jerusalem is considered Islam's third holiest city after Mecca and Medina. Among Muslims of an earlier era it was referred to as al-Bayt al-Muqaddas; later it became known as al-Quds al-Sharif. In 638 the Islamic Caliphate extended its dominion to Jerusalem. With the Arab conquest, Jews were allowed back into the city. The Rashidun caliph Umar ibn al-Khattab signed a treaty with Monophysite Christian Patriarch Sophronius, assuring him that Jerusalem's Christian holy places and population would be protected under Muslim rule.[55] Umar was led to the Foundation Stone on the Temple Mount, which he cleared of refuse in preparation for building a mosque. According to the Gaullic bishop Arculf, who lived in Jerusalem from 679 to 688, the Mosque of Umar was a rectangular wooden structure built over ruins which could accommodate 3,000 worshipers. The Umayyad caliph Abd al-Malik commissioned the construction of the Dome of the Rock in the late 7th century. The 10th century historian al-Muqaddasi writes that Abd al-Malik built the shrine in order to compete in grandeur with Jerusalem's monumental churches. Over the next four hundred years Jerusalem's prominence diminished as Arab powers in the region jockeyed for control.