But they didn't score three goals - the referee gave them the third goal.
The defence held their line, and Terry was in an offside position when he put the ball in the net.
The defence acted correctly - and the the linesman was at fault when there wasn't a whistle.
It could have been worse. When Roberto was at Wigan, Chelsea were allowed two offside goals. Time for video replays, I think.
Goals conceded (per match) 32 (1.45) Per game