Ok, given that the exercise is to ensure that we have 3 letter codes for all sets, I've suggested a few for those not specified elsewhere (Wikipedia or otherwise). Those new or changed ones I've put in bold.
OLD NEW DESCRIPTION
A LEA Limited Edition Alpha
B LEB Limited Edition Beta
U 2ED Unlimited
R 3ED Revised Edition
4E 4ED Fourth Edition
5E 5ED Fifth Edition
6E 6ED Classic (Sixth Edition)
7E 7ED Seventh Edition
8E 8ED Core Set - Eighth Edition
9E 9ED Core Set - Ninth Edition
10E 10E Core Set - Tenth Edition
AN ARN Arabian Nights
AQ ATQ Antiquities
LG LEG Legends
DK DRK The Dark
FE FEM Fallen Empires
HL HML Homelands
IA ICE Ice Age
AL ALL Alliances
CS CSP Coldsnap
MI MIR Mirage
VI VIS
VisionsWL WTH
WeatherlightTE TMP Tempest
SH STH Stronghold
EX EXO Exodus
US USG
Urza's Saga
UL ULG
Urza's Legacy
UD UDS
Urza's Destiny
MM MMQ Mercadian Masques
NE NMS Nemesis
PY PCY
ProphecyIN INV Invasion
PS PLS Planeshift
AP APC
ApocalypseOD ODY Odyssey
TO TOR
TormentJU JUD Judgment
ON ONS
OnslaughtLE LGN Legions
SC SCG Scourge
MR MRD Mirrodin
DS DST Darksteel
FD 5DN Fifth Dawn
CHK CHK Champions of Kamigawa
BOK BOK Betrayers of Kamigawa
SOK SOK Saviors of Kamigawa
RAV RAV Ravnica: City of Guilds
GP GPT Guildpact
DIS DIS Dissension
TSP TSP
Time SpiralTSB TSB
Time Spiral Timeshifted
PLC PLC
Planar ChaosFUT FUT
Future SightLRW LRW Lorwyn
MOR MOR
MorningtideSHM SHM Shadowmoor
EVE EVE Eventide
ALA ALA Shards of Alara
CH CHR Chronicles
AT ATH Anthologies
BR BRB Battle Royale
BD BTD Beatdown
DM DKM Deckmasters
EVG EVG Duel Decks: Elves vs. Goblins
DRB FVD From the Vault: Dragons
JVC JVC Duel Decks: Jace vs. Chandra
PT POR Portal
P2 P02 Portal Second Age
P3 PTK Portal Three Kingdoms
ST S99 Starter 1999
S2K S00 Starter 2000
AS
AST Astral
MED MED Masters Edition
ME2
ME2 Masters Edition II
UG UGL Unglued
UNH UNH Unhinged
TK
TKN Tokens
CST CST Coldsnap Theme Decks
BCE BCE Collector's Edition
4EA 4EA Fourth Edition (Alternate)
I2P I2P Introductory Two Player: Kazz & Zakk
MV
MUV Multiverse
DC
SDC Dreamcast (Sega)
BT8 BT8 Box Toppers 8E
BTP BTP Box Toppers Promo
FI
FIL Filler Cards
OV
OV6 Oversized 6x9
12
OV9 Oversized 9x12
WP
WPM Misprints-Wrong Pics
TP
TPR Test Prints
VG
VGD Vanguard
VGM VGM Vanguard (MTGO)
ARE ARE Promo –
Arena League
CHP CHP Promo – Champs
FNM FNM Promo – Friday Night Magic
GTW GTW Promo – Gateway
JGC JGC Promo – Judge Gift Cards
JSS JSS Promo – Junior Super Series, Junior Series, Junior APAC Series, Magic Scholarship Series
REW REW Promo – Magic Player Rewards
PRE PRE Promo – Prerelease
REL REL Promo – Release
PR
PRO Other Promos
ARC ARC Armada Comics Promos
DLM DLM DCI Legend Membership
HHL HHL Happy Holidays Promos
HPB HPB Harper Prism Books Promos
SUM SUM Summer of Magic Promos
THG THG
Two-Headed GiantSM
SMM Summer Magic
APL APL Asia Pacific Land Program
EUL EUL European Land Program
GUR GUR Guru Lands
MPS MPS Magic Premiere Shop Lands
REW REW Promo – Magic Player Rewards
PRE PRE Promo – Prerelease