Originally Posted by velocipedio
the problem with a triple is that it can be very difficult to tune your shifting for two reasons: (1) the long throw of the front derailleur compromises the chainline, (2) the long cage of the rear derailleur and the extra chain makes for sloppy rear shifting.
Just to add to this. A big factour is chainstay length. Bikes with extremely short chainstays as on many performance roadbikes these days will have a hard time accomodating a triple due to the high angles of deflection of the chainline. There is a higher chance of throwing the chain when going for that bailout gear too. This IMHO negates many advantages of a triple since you have to be very finesse in your shifting at the moment it's really critical to just get it in the right gear.