The Southsiders, or Sur Thirteen (XIII in roman numerals, hence X3) are a California-based gang with a presence in several Southwest states including Colorado and...Hawai'i? wtf??
Anyway, their colors are blue and gray, their main rival is Norte XIV or the Northsiders. The reason Surenos use X3 is M is the thirteenth letter of the alphabet in spanish...i think..... the M stands for the MExican Mafia, a prison gang which is older than crips, bloods, and alot of other gangs! It dates from about the 1940's I think.
Anyway, Norte X4 and Sur X3 aren't really rivals cause the Nortenos are from NorCal and rep red and Surenos are from SoCal and rep blue, it actually goes back to the MExican Mafia in prison. The MExican Mafia is rivals with a prison gang called Nuestra Familia, Our Family in Spanish, which is way nortenos rep the N and F, and Nuestra Familia owns the Nortenos, like la eMe (mexican mafia) owns Southsiders.
So, Nortenos call Surenos skraps and Surenos call Nortenos busters. Nortenos rep 4 and 14 while Surenos rep 3 and 13. Notably, the MS-13, a BRUTAL Hispanic gang (from EL SALVADOR not MEXICO, for you retards) uses the number 13...they basically stole it from the Surenos...who they don't get along with too well.
If you see Sur X3 spraypainted on some fence in the hood, it's the Southsiders...they're in most major cities in the western USA by now. Do not confuse with MS-13, a far scarier gang from El Salvador (Mara Salvatruchas means something like Gangster Salvadoran Soldiers....)