For TV Series is Three the Magic Number or a Curse?

I've been thinking about this a lot lately and that doesn't seem to be the case when it comes to television series.  If a show is not canceled by it's third season (Arrested Development, Veronica Mars) then it has peaked (Entourage, Weeds, Nip/Tuck).  So why is this?  I thought three was the magic number not the downfall of television shows number.  I feel as though these things happen for a couple different reasons.  A reason shows are absolutely wonderful in their third seasons is the chemistry that exist by the cast at this point.  Also, the writers have developed these characters so well by this point that the show is more cohesive and pulls us in more.  So why are shows canceled at this point...in my opinion it's solely marketing.  Time slots are the biggest killers of amazing series.  Anything usually airing on a Friday night is the biggest killer of tv shows and any newer show competing against a well established show is a goner as well. I mean, it's like David versus Goliath.   I think the biggest thing to figure out is how does a show stay fresh and interesting after that third season peak.  I think one way to avoid the curse of the three is to constantly evolve characters.  Just like real people change and grow so should a shows characters.  To stunt their intellectual and emotional maturity is to make the show unrealistic.  Writing tends to be the core of a thriving series despite how good the acting is if there is nothing good being said well then, who cares?  Unfortunately, this isn't the 80's anymore where a quest spot or adding a new character can instantly help a show gain another season.  It's a new time and new generation, so try harder tv peeps.