When spawned in GTA San Andreas, they appear as the Varrios Los Aztecas. They can't be spawned in GTA III.
The gang doesn't appear in the game outside of using cheats or mods. The gang can be found by entering a cheat.
As noted above, the gang members of the Unused Gang use the same character models from the Varrios Los Aztecas, with no changes in their external look. Their behavior, however, is different; they won't attack Carl at first sight, nor will they confront him if he strays too far inside their gang area. The vehicles they drive are Sentinels.