Where can one find the listing of the 3-call letters for all of the altcoins? There's BTC, and everything else.  But how are the call letters registered and is there a comprehensive list of them all?
As said above, there's no real, official "standard" everybody follows. Now then, you can go have a look at different exchanges and see what the call them. Cryptsy is famous for allowing you to deal with dozens of crypto-currencies, and they're constantly adding new ones (as well as taking out those deemed defunct) - you could go to their home-page and download 

Be warned, though, that not all call-signs are 3 letters-long, though. Dogecoin appears as "DOGE" in most of the exchanges it appears at, and forty-two-coin is usually represented as just "42".

Some call-signs, off Cryptsy's page - notice the 4-lettered ones, the non-obvious (as eMark being "DEM" or Primecoin being "XPM") and the potentially confuse (as "LK7" and "LKY")

42   42Coin
ALF  AlphaCoin
AMC  AmericanCoin
ANC  AnonCoin
ASC  AsicCoin
BET  Betacoin
BQC  BBQCoin
BTC  Bitcoin
BTE  ByteCoin
BTG  BitGem 
BUK  CryptoBuck
CAP  BottleCaps
CAT  CatCoin
CENT Pennies
CGB  CryptogenicBullion
CMC  Cosmoscoin
CNC  CHNCoin
COL  ColossusCoin
CSC  CasinoCoin
DBL  Doubloons
DEM  eMark
DGC  DigitalCoin    
DOGE Dogecoin
DVC  DevCoin
EAC  EarthCoin
ELC  ElaCoin
ELP  ElephantCoin   
EMD  Emerald    
EZC  EZCoin 
FFC  FireflyCoin
FLO  FlorinCoin 
FRC  FreiCoin   
FRK  Franko  
FST  FastCoin   
FTC  FeatherCoin    
GDC  GrandCoin
GLC  Globalcoin
GLD  GoldCoin   
GLX  Galaxycoin
GME  GameCoin   
IFC  InfiniteCoin   
IXC  IXCoin 
KGC  KrugerCoin
LK7  Lucky7Coin
LKY  LuckyCoin
LOT  LottoCoin
LTC  LiteCoin
MEC  MegaCoin
MEM  MemeCoin
MNC  MinCoin
NAN  NanoToken
NBL  Nibble 
NEC  NeoCoin
NET  Netcoin
NMC  NameCoin
NRB  NoirBits
NVC  NovaCoin
PHS  PhilosopherStone
PPC  Peercoin   
PTS  ProtoShares    
PXC  PhoenixCoin
QRK  Quark
RED  RedCoin    
RYC  RoyalCoin  
SBC  StableCoin 
SXC  SexCoin
TGC  TigerCoin  
TRC  TerraCoin  
UNO  Unobtanium 
WDC  WorldCoin  
XNC  XenCoin    
XPM  PrimeCoin  
YBC  YBCoin  
ZET  ZetaCoin