In the US, it's stated to arrive for domestic 2-3 or at least 3 hours before flight time. I am totally baffled at this recommendation. I understand that each person passing through will be delayed a little more at the checkpoint, but overall the line just gets longer by having more people there earlier. I think that if you have more people arriving earlier, there will just be larger crowds and makes more people late, so they tell us to arrive yet earlier?
Italian airlines said the same, 3 hours before departure..
Here in Canada too we have to be at the airport 3 hours beforehand. I think it's because people will always arrive late, so this way they'll be there at least 2 hours beforehand. Although I did hear of someone being refused entry in Paris when they showed up 2 1/2 hours early instead of 3.
In Amsterdam, they only request 3 hours for intercontinental flights. For european ones 2 hours will do. And besides that: nobody will fine you or anything if you show up one hour before take off. You will only be unable to choose a seat (and thus end up above the wing, or at the back near the toilets)
Most airlines close check-in desks some 40 minutes before departure, so that is the absolute latest you should arrive, when you have luggage to check-in that is. Sometimes I make short flights in europe with only handluggage (back in the old days, when handluggage was still allowed ). I checked in online, so there was no need to stand in the line. If you arrive at the gate just before it closes, there is nobody who will say anything.
Of course airlines want you to be there way in time, so they know for sure you're there and you will be on the flight, but there's no absolute nessacity, and they can't force you or punish you for not complying with the -stupid- request of spending two boring hours for nothing at the airport.
Personally I like to allocate my own time, so I'll arrive when I want to (of course in time, but not ridiculously early...)
I've always been asked to arrive two hours early, but have never been refused entry even if I come later than recommended - though I usually try to come early to avoid queues and get a good seat. Of course, you probably won't be able to get the seat you desired if you arrive late, but so far I've not encountered my flight being held up because of late passengers.
3 hours before the flight is a bit... too early.