The "X" is a surname-like identifier given to members of the Nation of Islam (NOI)--the organization that all of these men were in. As for the number, those were given out based on one's first name. In each city, the NOI has a temple/mosque and the first person who joined that temple with a particular first name was simply given the "surname" X. So the first John at the NY temple was given the name John X. The second John who joined was given the name John 2X. Etc. So Norman 3X means he was the third Norman to join his temple (which I believe was Newark's temple, No. 25). Now, because the naming system was temple-based, people could be from different temples and have the same name. So for instance, in the 50s and 60s there were 2 prominent NOI figures named John 3X, one from Chicago and one from Newark.

As for the why they were given the X name... First of all, you should know that when the NOI started in the 1930s, almost no one in the group got an X. Instead, the NOI founder, WD Fard, personally gave members more traditional sounding Muslim surnames, like Sharreiff, Karreim, and Muhammad. That is why the NOI's later famous leader Elijah Muhammad, who joined in the early 30s, has that surname. In the 1940s, it was explained that the reason the X started being used was because the NOI believed that only Fard himself could give out a "righteous" name, so after his departure in 1934 new members were supposedly given the X name as a placeholder, X being the standard placeholder (ie variable) in mathematics, and it was expected that Fard would one day resturn as a Messiah in an Armageddon and give all the people with Xs their "righteous" names. (This connecting of the X with Armageddon, by the way, suggests that the X name was looked upon by early members as having deep cullture meaning, since black folk religion since the 19th c contained the idea that the symbol X was a sign of black people and their expected redemption from oppression, as foretold [they said] in the 7th chapter of the Book of Revelation. The X was therefore seen as a magical, important symbol.) But It was also said that even in the early days, when new members were waiting to hear back from Fard with their new names, at that time they too were given an X, so the Armageddon stuff came later. However, that 1940s report seems to imply that even after Fard left, for a few years his ministers (like Elijah Muhammad) had the ability to give righteous names too. And in fact when dozens of NOI members were arrested in the 1940s and had their names appear in the newspapers, NOT ONE had the surname of X (although 1 FBI report from that time does mention someone using it, and oral reports attest to it.) So it seems the group was rather inconsistent at about the X name at that time.

However, upon the inprisonment of many members in the 40s, Elijah Muhammad started changing and standardizing many NOI practices, including the giving of the X. Thus, by 1950 pretty much all new members were consistently given the X name. And in the 1960s, new explanations had appeared for its use. Now it was emphasized that the X stood for the unknown, just as the African family names of the slaves were unknown, and the X served to replace the "slave name" --ie the surnames African Americans had received from slavemasters. (By the way, the earliest recorded black Muslim use of the term "slave name" comes not from NOI members, but from Members of the Moorish Science Temple, which preceded and influenced the NOI.)

But, that is still not the whole story. Even after Elijah Muhammad's reforms, he still sometimes gave his ministers the surname Shabazz (which is the name of the tribe, according to NOI theology, from which African Americans are decended.) And sometimes, despite it going against the official NOI rules, members gave themselves and their kids traditional Muslim names, first and last. This is the original of the surname of the famous rapper Tupac Shakur. The Shakur name was adopted by certain NOI Muslims from the same family in New York. These Muslims were well respected followers of Malcolm, so when Malcolm died and they joined the Black Panthers, other people they knew took the Shakur name out of respect for that family (eg the notorious Assata Shakur). Tupac's mom Afeni had a relationship with a Shakur, which is how she got the name, plus she was close to other Shakurs, including Assata.

After Elijah died in 1975, his son Wallace dissolved the NOI, made the community Sunni, and therefore abandoned the use of the X, letting everyone take Muslim names. Louis Farrakhan (who had previously gone as Louis X) revived the NOI in 1978 and therefore the use of X, although I think he gives some people righteous names.

Sources: Bontemps and Conroy, They Seek a City: Bontemps and Conroy, Any Place but Here; Muhammad, Dear Holy Apostle; Bowen, History of Conversion to Islam in the United States, volume 2