I resorted the acronym list we host and pulled out the three letter acronyms. Dave's list above has some good ones that aren't in the acronym list.

ABS
ADC
ADX
AFR
AIR
AIT
AWG
BBL
BCC
BCM
BDC
BIN
BLM
BPC
BPW
BTU
C/L
CAN
CAT
CCI
CCP
CDI
CEL
CFI
CFI
CFI
CID
CKP
CLT
CMP
CNP
CPU
CTS
DAC
DEF
DIS
DRP
DVM
ECM
ECT
ECU
EEC
EFI
EGO
EGR
EGT
ESC
EST
FAR
FED
FPR
GPS
HEI
I/O
IAC
IAT
ICE
IGN
INT
ION
IPW
ISC
kPa
KPH
LCD
LSA
MAF
MAP
MAP
MAT
MCU
MDP
MFI
MIL
MPG
MPH
MPI
NEU
OBD
OEM
OTS
P/N
P/S
P+H
PCM
PCV
PPH
PPM
PSI
PWM
QDM
RAM
RAP
RFI
ROM
RPM
RTV
RWD
SAE
SBC
SES
SFI
TAC
TBI
TBI
TCC
TCM
TDC
TFT
TPI
TPS
TRC
TSA
TTS
TVS
VAC
VIN
VSS
WOT
XDF