Actually, it's a bit more complicated than that. 274 was the first issue that was completely 3E. 272 & 273 each had a 3E article (Stats for Tiamut and Bahamut & the Sembia Seven NPC stats respectively). Also, each issue for the year leading up to 274 had a "Countdown to 3E" article (which actually are of little practical use now).
Glyfair of Glamis