There were actually four series.
DMC, DMC2, DMC3, and DMC3 Special edition.
Customers complained that DMC was too hard, so in DMC3 Special Ed, they made a lesser Difficulty, and made somethings better.
DMC3 is now a collector's item as it is now not in production
BTW, the storyline goes like this:
DMC3>DMC1>DMC2.