A triple-aisle airplane wouldn't necessarily be too big to fit into airports, as that really depends on weight, wingspan and length, not on how wide the fuselage is. As far as evacuation, I honestly don't know. IIRC, the design I saw had 6 Type A doors on either side, and I believe I read that they had "double-wide" evac slides. In other words, 2 passengers could go in quick succession. I don't know if I am remembering this correctly or not, so maybe someone could help me out. I imagine if such a system exists, the A3XX will probably have it too.
From what I've heard, the biggest problem with double-decker planes is:
A) cargo space. Remember that cargo is very important to airline revenue. The more people you put on a plane, the less cargo room (i.e. more baggage) is available in the hold. Since adding another passenger deck doesn't add another cargo deck, you sacrifice alot of additional revenue. However, Airbus has an excellent record on planes with great cargo carrying abilities, so I'm sure they have this sorted out.
B) evacuation worries. This is two-fold. One issue is the amount of time it takes to evacuate that many people. However, this will be a problem on any SuperJumbo, and is not limited to double-deckers. The other side is possible injuries sustained when evacuating from the upper deck. The next time there is an emergency on any plane, listen carefully. More often than not, nobody gets injured during the incident. However, some fool always manages to break a leg or something going down the slide.
This will be more of a problem when sliding a greater distance (i.e. more speed + harder stop at the end).
The Boeing model I saw eliminates some of these problems. Obviously, the time to evacuate would still be an issue. But since it is only one deck, you would have a standard length slide. The real advantage, however, is lower cargo hold volume. The cross section of the fuselage is an ovoid (egg). Therefore, not only do you have 3-aisles, but the hold accomodates standard LD-3 containers 3 abreast. In fact, the report I read stated that this design has the same volume in its lower hold that a 744F has on its main deck!
Sorry for the long post. I got a little carried away.
All gave some. Some gave all.