I'm too lazy to look up right now just how many 
series I have multiple cels from, but I think 
it's pretty safe to say it's more than ten. ^_~

So... three huh?  I'm glad I'm not so limited in 
real life.  I would probably pick:

Utena - perhaps my favorite series to collect 
from, I'm a big fan of the show and movie, I love 
the characters, the art and the scenes.

Violinist of Hameln - very close to Utena 
collecting, I can say the same of the show, 
movie, characters, art and scenes. ^_^

Now, the last one is tough.  If I only collected 
from three, I would have more money to put 
towards each series.  This opens up some 
possibilities I might not have afforded to collet 
as seriously.  Battle Athletess is tempting but 
I'm not sure I'd want to duke it out weekly at 
Animaxis ^_~.  Trigun is tempting because I wuv 
Wolfwood, but can the man hold the series by 
himself?  Noir and Cowboy Bebop get to be 
finalists too.  It's hard to pick between a 
gorgeous but short movie like Angel's Egg and a 
long running series like Patlabor...  *gives up 
and says:

Trigun - at least, that's what it would be right 
at this moment ^_~

Keys
Can't I count Misc as a series?  I have it as a 
section in my gallery after all ^_^