MMC3 pinout

Nintendo MMC3: 44-pin QFP (Canonically mapper 4)

                             / \
                            / O \
                    n/c -- /01 44\ -> CHR A16 (r)
           (r) CHR A10 <- /02   43\ -> CHR A11 (r)
          (n) PPU A12 -> /03     42\ -> PRG RAM /WE (w)
         (n) PPU A11 -> /04       41\ -> PRG RAM +CE (w)
        (n) PPU A10 -> /05         40\ -- GND
               GND -- /06         \ 39\ <- CPU D3 (nrw)
      (r) CHR A13 <- /07           \ 38\ <- CPU D2 (nrw)
     (r) CHR A14 <- /08         \  _\ 37\ <- CPU D4 (nrw)
    (r) CHR A12 <- /09         \ \|    36\ <- CPU D1 (nrw)
 (n) CIRAM A10 <- /10           \ \     35\ <- CPU D5 (nrw)
  (r) CHR A15 <- /11         \  _\    o  34\ <- CPU D0 (nrw)
  (r) CHR A17 <- \12       /\ \|         33/ <- CPU D6 (nrw)
      (n) /IRQ <- \13        \ \        32/ <- CPU A0 (nrw)
    (n) /ROMSEL -> \14   /    \        31/ <- CPU D7 (nrw)
             GND -- \15  |_   /       30/ -> PRG RAM /CE (w)
              n/c -- \16   |         29/ <- M2 (n)
           (n) R/W -> \17  |/       28/ -- GND
        (r) PRG A15 <- \18         27/ -- VCC
         (r) PRG A13 <- \19       26/ -> PRG /CE (r)
          (n) CPU A14 -> \20     25/ -> PRG A17 (r)
           (r) PRG A16 <- \21   24/ <- CPU A13 (n)
            (r) PRG A14 <- \22 23/ -> PRG A18 (r)
                            \ O /
                             \ /

01, 16: both officially no connection. sometimes shorted to pin 02, 15 respectively

Note the orientation of the text: "MMC3" when viewed upright specifies pin 1 is bottom face, leftmost.

Mappers 37 and 47 connect pins 42 and 30 to a 74161

iNES Mapper 118 connects pin 12 to CIRAM A10, and pin 10 is n/c.

iNES Mapper 119 connects pin 44 to a 7432 and to the CHR RAM's +CE pin.

Pirate versions (600 mil 40-pin DIP package)

            .--\/--.                                 .--\/--.               
      M2 -> |01  40| -- +5V           CPU !ROMSEL -> |01  40| -> WRAM !CE   
WRAM !CE <- |02  39| -- NC                PRG !CE <- |02  39| -- +5V        
  CPU D7 -> |03  38| -> PRG !CE          WRAM !WE <- |03  38| -> WRAM CE       
  CPU A0 -> |04  37| -> PRG A17           CPU A14 -> |04  37| <- CPU R/!W   
  CPU D6 -> |05  36| <- CPU A13           CPU A13 -> |05  36| -> PRG A13    
  CPU D0 -> |06  35| -> PRG A18            CPU A0 -> |06  35| -> PRG A14    
  CPU D5 -> |07  34| -> PRG A14                M2 -> |07  34| -> PRG A15    
  CPU D1 -> |08  33| -> PRG A16           PPU A12 -> |08  33| -> PRG A16    
  CPU D4 -> |09  32| <- CPU A14              !IRQ <- |09  32| -> PRG A17    
  CPU D2 -> |10  31| -> PRG A13         CIRAM A10 <- |10  31| -> PRG A18    
  CPU D3 -> |11  30| -> PRG A15           PPU A10 -> |11  30| -- NC         
 WRAM CE <- |12  29| <- CPU R/!W          PPU A11 -> |12  29| -> CHR A17    
WRAM !WE <- |13  28| <- CPU !ROMSEL        CPU D0 -> |13  28| -> CHR A16    
 CHR A11 <- |14  27| -> !IRQ               CPU D1 -> |14  27| -> CHR A15    
 CHR A16 <- |15  26| -> CHR A17            CPU D2 -> |15  26| -> CHR A14    
 CHR A10 <- |16  25| -> CHR A15            CPU D3 -> |16  25| -> CHR A13    
 PPU A12 -> |17  24| -> CIRAM A10          CPU D4 -> |17  24| -> CHR A12    
 PPU A11 -> |18  23| -> CHR A12            CPU D5 -> |18  23| -> CHR A11    
 PPU A10 -> |19  22| -> CHR A14            CPU D6 -> |19  22| -> CHR A10    
     GND -- |20  21| -> CHR A13               GND -- |20  21| <- CPU D7     
            `------'                                 `------'               
           AX5202P #1                             AX5202P #2 (NTDEC?)       

Notes:

• Both versions enables WRAM at $6000-$7fff at power up but protects them from writes (during CPU write cycle to $6000-$7fff when WRAM is protected, WRAM !CE and WRAM CE are not asserted). When RAM is disabled, open bus behavior is observed.
• NC seems to be not connected internally in both versions (multimeter diode test does not show any conducting voltage between NC and any other pins)
• First version is the one you can still buy nowadays (it has AX5202P marking).
• Second version was found in at least one game - Doki Doki Yuuenchi bootleg (the chip does not have any markings)