cbloom,
When you fail to complete a full 3x5, there's a bit of decision-making required as to whether you should reset the weights (drop weights by 10-35 lbs.), use the same weight, or just keep pushing forward. Thankfully, kethnaab on the bb.com forums addressed this in great detail
here.
Basically: If you drop the last rep of your third set, I wouldn't worry about it too much. If you end up doing 5/4/4 with some struggling, I'd keep the weight the same or do a tiny weight jump. If it doesn't get better in the next few workouts or even gets worse, your muscles need a bit of a break so go ahead and reset. If you put up a stinker like 5/3/3, you'll have to make a decision whether it was just a bad workout (poor pre-workout nutrition, hung over, not focused) or whether you're in over your head. If it might be an aberration, try again next workout. If it still feels like [censored] on the next workout, drop the weight down.
Edit: Also, if I drop a rep or two on a set, I'll take an extra long time before I hit the next set -- somewhere between 3-4 minutes.
Brag - set a PR by squatting 215 yesterday.
Beat - It was 5/4/4 and I ended up having to drop the bar on the second set and crawl out from the power rack.
Variance - Indian guys next to me using the Smith machine to do 1/8 squats with 3 plates. Roar.