The premise for SR is simple: There are three gangs.  You are recruited by a fourth to annihilate the other three.  Kill the police.  Do non-linear missions, win.
The premise for SR2 is simple: There are four gangs.  You are the leader of one.  Annihilate the other three.  Kill the police.  Do non-linear missions, win.
The premise for SR:TT is simple: There are two gangs.  You are the leader of one.  The other one is divided into...three sets.  Annihilate the three sets.  Kill the police.  Do semi-linear missions, win.
Very similar games, all fun as sh!t.